cleanup: Remove unused isFunction (#71838)

Related to https://github.com/getsentry/frontend-tsc/issues/13

static/app/utils.tsx

@@ -290,8 +290,6 @@ export function generateQueryWithTag(prevQuery: Query, tag: EventTag): Query {
   return query;
 }
 
-export const isFunction = (value: any): value is Function => typeof value === 'function';
-
 // NOTE: only escapes a " if it's not already escaped
 export function escapeDoubleQuotes(str: string) {
   return str.replace(/\\([\s\S])|(")/g, '\\$1$2');