ci: remove outdated default versions for ABI check

ci: remove outdated default versions for ABI check

The variables REF_GIT_TAG and LIBABIGAIL_VERSION are set
in the CI configuration like .travis.yml or .github/workflows/build.yml.
The default values are outdated and probably unused.

The default values are removed completely
to avoid forgetting an update in future.

The use of the variables is quoted to make sure
a missing value will trigger an appropriate failure.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Aaron Conole <aconole@redhat.com>
Acked-by: David Marchand <david.marchand@redhat.com>