Change checkpatch.conf after migration to tf.org

A specific checkpatch setting was used because of GitHub. This necessity
doesn't exist anymore.

Change-Id: Ie2225a5cb88654f3b7407e43e0a48fafa9a9165c
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>

.checkpatch.conf

@@ -43,10 +43,6 @@
 # Commit messages might contain a Gerrit Change-Id.
 --ignore GERRIT_CHANGE_ID
 
-# Do not check the format of commit messages, as Github's merge commits do not
-# observe it.
---ignore GIT_COMMIT_ID
-
 # FILE_PATH_CHANGES reports this kind of message:
 # "added, moved or deleted file(s), does MAINTAINERS need updating?"
 # We do not use this MAINTAINERS file process in TF.