remove DW_TAG_mutable_type

Based on patch for dwarves:

https://git.kernel.org/cgit/devel/pahole/pahole.git/commit/?id=943a0de0679a34b5e630f85cd01cca35c0d0e544

DW_TAG_mutable_type was a mistake in an early DWARFv3 draft and was
removed in the final version.

Signed-off-by: Marcin Juszkiewicz <mjuszkiewicz@redhat.com>