rm mwext whitespaces jobs

The idea was to fail whenever we have trailing whitespace using 'git
diff --check' but that is too unreliable and often will fail on i18n
files.  So just get rid of the jobs definitions, we can later use phpcs
to achieve the same goal.

Change-Id: Ibb7df0d595f7fe197de4318ef684f66faac36c8e