chore(weed/storage/backend/s3_backend): remove unused function (#9715)

* chore(weed/storage/backend/s3_backend): remove unused function

* fix(s3_backend): cache session under the composite region|endpoint key

createSession looked up sessions by region|endpoint but stored them by
region alone, so the cache never hit and a new session was built every
call. With getSession gone the lock can also drop to a plain Mutex.

---------

Co-authored-by: Chris Lu <chris.lu@gmail.com>

weed/storage/backend/s3_backend/s3_sessions.go

@@ -15,17 +15,9 @@ import (
 
 var (
 	s3Sessions   = make(map[string]s3iface.S3API)
-	sessionsLock sync.RWMutex
+	sessionsLock sync.Mutex
 )
 
-func getSession(region string) (s3iface.S3API, bool) {
-	sessionsLock.RLock()
-	defer sessionsLock.RUnlock()
-
-	sess, found := s3Sessions[region]
-	return sess, found
-}
-
 func createSession(awsAccessKeyId, awsSecretAccessKey, region, endpoint string, forcePathStyle bool) (s3iface.S3API, error) {
 
 	sessionsLock.Lock()
@@ -56,7 +48,7 @@ func createSession(awsAccessKeyId, awsSecretAccessKey, region, endpoint string,
 
 	t := s3.New(sess)
 
-	s3Sessions[region] = t
+	s3Sessions[cacheKey] = t
 
 	return t, nil