Remove cleanups from mdebugread.c

Remove cleanups from mdebugread.c

This removes the remaining cleanups from mdebugread.c, replacing them
with gdb::def_vector.

Tested by the buildbot, though I doubt this exercises mdebugread.

gdb/ChangeLog
2018-05-11  Tom Tromey  <tom@tromey.com>

	* mdebugread.c (parse_partial_symbols, psymtab_to_symtab_1): Use
	gdb::def_vector.