Remove MINIO_DEBUG environment variable (#2794)

Removes the unimplemented settings of MINIO_DEBUG=mem and makes
MINIO_DEBUG=lock the default behaviour.

cmd/namespace-lock_test.go

@@ -298,14 +298,11 @@ func TestLockStats(t *testing.T) {
 		},
 	}
 	var wg sync.WaitGroup
-	// enabling lock instrumentation.
-	globalDebugLock = true
 	// initializing the locks.
 	initNSLock(false)
 
 	// set debug lock info  to `nil` so that the next tests have to initialize them again.
 	defer func() {
-		globalDebugLock = false
 		nsMutex.debugLockMap = nil
 	}()