chore(alerts): remove migration email templates (#62461)

Part of #60356

src/sentry/templates/sentry/emails/alert_migration.html

@@ -1,63 +0,0 @@
-{% extends "sentry/emails/base.html" %}
-
-{% load sentry_assets %}
-{% load i18n %}
-
-{% block main %}
-<h3>Performance based alerts just got a lot more accurate.</h3>
-
-<p>
-    Starting from November 30th, alerts will be based on <strong>all the transaction events you send to Sentry</strong>,
-    not just
-    <a href="https://docs.sentry.io/product/performance/retention-priorities/">the ones that are stored.</a>
-</p>
-
-<h4>How does this affect me, and my team?</h4>
-
-<p>
-    Because we're potentially extracting metrics from a significantly larger number of events, existing thresholds could
-    result in a spammy experience. Instead of waiting until November 30th, we recommend that you take action now by
-    reviewing the thresholds of each affected alert. This will allow you to migrate alerts ahead of time with the
-    assurance that they will work as expected.
-</p>
-
-<img src="{% absolute_asset_url 'sentry' 'images/email/alert-migration-lgtm.png' %}" width="100%"/>
-
-<h4>How do I review my thresholds?</h4>
-
-<p>
-    It's pretty simple really, but unfortunately, also pretty tedious (sorry about that)...
-</p>
-<p>
-    We’ve marked all your affected alerts with a "⚠" sign, so that you can quickly identify the ones that need your
-    attention. In order to perform the review:
-</p>
-<ul>
-    <li>open each rule marked with a "⚠" sign and click “Review Thresholds”</li>
-    <li>verify whether current thresholds are still valid or make adjustments as needed</li>
-    <li>save the changes to finish the review</li>
-</ul>
-
-<p>
-    To get the ball rolling, we've included a shortlist of alerts that need review below, but click on "View Performance
-    Alerts" to see the list of affected ones in Sentry.
-</p>
-
-<p><a href="{{ alerts_url }}" class="btn">View Performance Alerts</a></p>
-
-<h5 class="header">Affected Alerts</h5>
-
-<ul>
-    {% for url, name in alerts %}
-    <li>
-        <a href="{{ url }}">{{ name }}</a>
-    </li>
-    {% endfor %}
-</ul>
-
-<hr/>
-
-<p>
-    If you need any assistance, please reply to this email or contact support@sentry.io.
-</p>
-{% endblock %}

src/sentry/templates/sentry/emails/alert_migration.txt

@@ -1,29 +0,0 @@
-Performance based alerts just got a lot more accurate
------------------------------------------------------------
-
-Starting from November 30th, alerts will be based on all the events you send to Sentry, not just the ones that are stored.
-
-How does this affect me, and my team?
-
-Because we're potentially extracting metrics from a significantly larger number of events, existing thresholds could
-result in a spammy experience. Instead of waiting until November 30th, we recommend that you take action now by
-reviewing the thresholds of each affected alert. This will allow you to migrate alerts ahead of time with the
-assurance that they will work as expected.
-
-How do I review my thresholds?
-
-It's pretty simple really, but unfortunately, also pretty tedious (sorry about that)...
-
-We’ve marked all your affected alerts with a "⚠" sign, so that you can quickly identify the ones that need your
-attention. In order to perform the review:
-
-- open each rule marked with a "⚠" sign and click “Review Thresholds”
-- verify whether current thresholds are still valid or make adjustments as needed
-- save the changes to finish the review
-
-To get the ball rolling, we've included a shortlist of alerts that need review below, but click on "View Performance
-Alerts" to see the list of affected ones in Sentry.
-
-You can view all of your impacted alerts here {{ alerts_url }}
-
-If you need any assistance, please reply to this email or contact support@sentry.io.