Always enable caches (#28527)

Nowadays, cache will be used on almost everywhere of Gitea and it cannot
be disabled, otherwise some features will become unaviable.

Then I think we can just remove the option for cache enable. That means
cache cannot be disabled.
But of course, we can still use cache configuration to set how should
Gitea use the cache.

modules/git/last_commit_cache.go

@@ -39,7 +39,7 @@ func NewLastCommitCache(count int64, repoPath string, gitRepo *Repository, cache
 	if cache == nil {
 		return nil
 	}
-	if !setting.CacheService.LastCommit.Enabled || count < setting.CacheService.LastCommit.CommitsCount {
+	if count < setting.CacheService.LastCommit.CommitsCount {
 		return nil
 	}