Drop the TARBALL_{COMPRESSOR,EXTENSION} variables

Drop the TARBALL_{COMPRESSOR,EXTENSION} variables

All currently supported BIND 9 branches use xz-packed tarballs for
source code distribution.  Having variables with lengthy names that only
hold 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_COMPRESSOR and TARBALL_EXTENSION
variables in .gitlab-ci.yml with a fixed string ("xz") to simplify the
contents of that file.

(cherry picked from commit 832c7d4396dcaf8ecaf417be92f7ef1023dc13a7)