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

It turns out that changing WARN_QA and ERROR_QA results in do_configure's QA
postfunc re-executing, so changing a QA test results in a complete rebuild.

This is just too much and the lesser evil of needing to do a full rebuild to
verify changed QA flags is preferable to an enforced full rebuild.

This reverts commit daecfc34.

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