bitbake.conf: don't remove WARN_QA and ERROR_QA from hashes

Changing WARN_QA and ERROR_QA should cause do_package_qa to re-execute, so
removing them from the sstate hashes is harmful.

They were added back when sanity testing was part of packaging and this was the
lesser evil, compared to changing sanity tests causing a re-package of
everything.

Signed-off-by: Ross Burton <ross.burton@intel.com>