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/cache/cache.go

@@ -24,11 +24,11 @@ func newCache(cacheConfig setting.Cache) (mc.Cache, error) {
 	})
 }
 
-// NewContext start cache service
-func NewContext() error {
+// Init start cache service
+func Init() error {
 	var err error
 
-	if conn == nil && setting.CacheService.Enabled {
+	if conn == nil {
 		if conn, err = newCache(setting.CacheService.Cache); err != nil {
 			return err
 		}

modules/cache/cache_test.go

@@ -22,9 +22,9 @@ func createTestCache() {
 }
 
 func TestNewContext(t *testing.T) {
-	assert.NoError(t, NewContext())
+	assert.NoError(t, Init())
 
-	setting.CacheService.Cache = setting.Cache{Enabled: true, Adapter: "redis", Conn: "some random string"}
+	setting.CacheService.Cache = setting.Cache{Adapter: "redis", Conn: "some random string"}
 	con, err := newCache(setting.Cache{
 		Adapter:  "rand",
 		Conn:     "false conf",