update: remove gcs storage driver build tags

GCS storage driver used to be conditionally built due to its being
outdated and basically unmaintained. Recently the driver has gone
through a rework and updates. Let's remove the build tag so we have less
headaches dealing with it and try keeping it up to date.

Signed-off-by: Milos Gajdos <milosthegajdos@gmail.com>

registry/storage/driver/gcs/doc.go

@@ -1,3 +1,2 @@
-// Package gcs implements the Google Cloud Storage driver backend. Support can be
-// enabled by including the "include_gcs" build tag.
+// Package gcs implements the Google Cloud Storage driver backend.
 package gcs

registry/storage/driver/gcs/gcs.go

@@ -1,6 +1,3 @@
-//go:build include_gcs
-// +build include_gcs
-
 // Package gcs provides a storagedriver.StorageDriver implementation to
 // store blobs in Google cloud storage.
 //

registry/storage/driver/gcs/gcs_test.go

@@ -1,6 +1,3 @@
-//go:build include_gcs
-// +build include_gcs
-
 package gcs
 
 import (