Revert "[FEAT]: Add mem caching to the API to create and retrieve IntegritySignature" (#27936)

This reverts commit c6192b8b40656c44ba0a89cdd569fb0c0e4f87c4.
The caching does little to save performance and in the case of whole
course interation, it has a netgative performance impact.

openedx/core/djangoapps/agreements/api.py

@@ -5,11 +5,9 @@ Agreements API
 import logging
 
 from django.contrib.auth import get_user_model
-from django.core.cache import cache
 from django.core.exceptions import ObjectDoesNotExist
 from opaque_keys.edx.keys import CourseKey
 
-from openedx.core.djangoapps.agreements.cache import get_integrity_signature_cache_key
 from openedx.core.djangoapps.agreements.models import IntegritySignature
 
 log = logging.getLogger(__name__)
@@ -35,9 +33,6 @@ def create_integrity_signature(username, course_id):
             'Integrity signature already exists for user_id={user_id} and '
             'course_id={course_id}'.format(user_id=user.id, course_id=course_id)
         )
-    cache_key = get_integrity_signature_cache_key(username, course_id)
-    # Write into the cache for future retrieval
-    cache.set(cache_key, signature)
     return signature
 
 
@@ -53,17 +48,10 @@ def get_integrity_signature(username, course_id):
         * An IntegritySignature object, or None if one does not exist for the
           user + course combination.
     """
-    cache_key = get_integrity_signature_cache_key(username, course_id)
-    cached_integrity_signature = cache.get(cache_key)
-    if cached_integrity_signature:
-        return cached_integrity_signature
-
     user = User.objects.get(username=username)
     course_key = CourseKey.from_string(course_id)
     try:
-        signature = IntegritySignature.objects.get(user=user, course_key=course_key)
-        cache.set(cache_key, signature)
-        return signature
+        return IntegritySignature.objects.get(user=user, course_key=course_key)
     except ObjectDoesNotExist:
         return None
 
@@ -78,12 +66,5 @@ def get_integrity_signatures_for_course(course_id):
     Returns:
         * QuerySet of IntegritySignature objects (can be empty).
     """
-
     course_key = CourseKey.from_string(course_id)
-    course_integrity_signature = IntegritySignature.objects.filter(
-        course_key=course_key
-    ).select_related('user')
-    for signature in course_integrity_signature:
-        cache_key = get_integrity_signature_cache_key(signature.user.username, course_id)
-        cache.set(cache_key, signature)
-    return course_integrity_signature
+    return IntegritySignature.objects.filter(course_key=course_key)