Drop the TARBALL_EXTENSION variable

Drop the TARBALL_EXTENSION variable

All currently supported BIND 9 branches use xz-packed tarballs for
source code distribution.  Having a variable with a lengthy name that
only holds two characters does not improve readability - it was only
useful for maintaining .gitlab-ci.yml consistency between BIND 9.11 and
all the newer branches, but that era has come to an end a while ago.

Replace all occurrences of the TARBALL_EXTENSION variable in
.gitlab-ci.yml with a fixed string ("xz") to simplify the contents of
that file.