dwarf.h: Remove non-existing DW_TAG_mutable_type.

dwarf.h: Remove non-existing DW_TAG_mutable_type.

The DW_TAG_mutable_type was only mentioned in an early draft of DWARFv3.
But was removed because there are no C++ mutable qualified types. It was
replaced by a new attribute DW_AT_mutable on DW_TAG_member DIEs. The new
attribute is available in dwarf.h.
http://dwarfstd.org/ShowIssue.php?issue=050223.1

DW_TAG_mutable_type was only used internally in some backends (which
just ignored it anyway).  dwarves did use it to turn it into a string
value, libabigail used it and ignored it (patches to remove sent).
GCC, GDB and binutils don't use nor define it.

Signed-off-by: Mark Wielaard <mjw@redhat.com>