ci/base: don't try to make warnings fatal

The generated local.conf has ERROR_QA=${WARN_QA} in an attempt to make
warnings fatal, but this appears to be just disabling some warnings and
error instead.

As we already have warning detection in the GitLab CI script, this is
redundant and can be removed.

Change-Id: I393874edbae148ee338a7069bbf800603c028242
Signed-off-by: Ross Burton <ross.burton@arm.com>