MM-56878: Remove unused store method (#26230)

IsUserInChannelUseCache wasn't being used anywhere.

https://mattermost.atlassian.net/browse/MM-56878

```release-note
NONE
```

server/channels/store/storetest/channel_store.go

@@ -3737,25 +3737,6 @@ func testChannelStoreGetChannels(t *testing.T, rctx request.CTX, ss store.Store)
 	require.Equal(t, o1.Id, list[0].Id, "missing channel")
 	require.Equal(t, o3.Id, list[1].Id, "missing channel")
 
-	require.True(
-		t,
-		ss.Channel().IsUserInChannelUseCache(m1.UserId, o1.Id),
-		"missing channel")
-	require.True(
-		t,
-		ss.Channel().IsUserInChannelUseCache(m1.UserId, o2.Id),
-		"missing channel")
-
-	require.False(
-		t,
-		ss.Channel().IsUserInChannelUseCache(m1.UserId, "blahblah"),
-		"missing channel")
-
-	require.False(
-		t,
-		ss.Channel().IsUserInChannelUseCache("blahblah", "blahblah"),
-		"missing channel")
-
 	ss.Channel().InvalidateAllChannelMembersForUser(m1.UserId)
 }
 

server/channels/store/storetest/mocks/ChannelStore.go

@@ -1903,20 +1903,6 @@ func (_m *ChannelStore) InvalidatePinnedPostCount(channelID string) {
 	_m.Called(channelID)
 }
 
-// IsUserInChannelUseCache provides a mock function with given fields: userID, channelID
-func (_m *ChannelStore) IsUserInChannelUseCache(userID string, channelID string) bool {
-	ret := _m.Called(userID, channelID)
-
-	var r0 bool
-	if rf, ok := ret.Get(0).(func(string, string) bool); ok {
-		r0 = rf(userID, channelID)
-	} else {
-		r0 = ret.Get(0).(bool)
-	}
-
-	return r0
-}
-
 // MigrateChannelMembers provides a mock function with given fields: fromChannelID, fromUserID
 func (_m *ChannelStore) MigrateChannelMembers(fromChannelID string, fromUserID string) (map[string]string, error) {
 	ret := _m.Called(fromChannelID, fromUserID)