refs(tests): Remove `USE_SNUBA` env variable from test configs. (#39316)

This is unused since we merged
https://github.com/getsentry/sentry/pull/39297. So both the backend and
snuba tests are running on almost the same config now. The main
difference is that snuba tests run kafka, while backend don't.

We could probably look into unifying these jobs in the near future.

.github/workflows/acceptance.yml

@@ -176,7 +176,6 @@ jobs:
           make run-acceptance
         env:
           PYTEST_SNAPSHOTS_DIR: ${{ steps.setup.outputs.acceptance-dir }}
-          USE_SNUBA: 1
 
       - name: Save snapshots
         uses: getsentry/action-visual-snapshot@e832d70549c14886ddc2ab809b436ba72e30e19b  # main

.github/workflows/backend.yml

@@ -102,8 +102,6 @@ jobs:
 
       - name: Run backend test (${{ steps.setup.outputs.matrix-instance-number }} of ${{ steps.setup.outputs.matrix-instance-total }})
         run: |
-          # Note: `USE_SNUBA` is not used for backend tests because there are a few failing tests with Snuba enabled.
-          unset USE_SNUBA
           make test-python-ci
 
       - name: Handle artifacts
@@ -373,7 +371,6 @@ jobs:
     env:
       # XXX: MATRIX_INSTANCE_TOTAL must be hardcoded to the length of strategy.matrix.instance.
       MATRIX_INSTANCE_TOTAL: 2
-      USE_SNUBA: 1
       MIGRATIONS_TEST_MIGRATE: 1
 
     steps: