Fix a memory leak and remove an unused member

Fix a memory leak and remove an unused member

I noticed that setup_type_unit_groups leaks the symtab vector -- it
allocates this with XNEWVEC, but from what I can tell, nothing frees
it.  This patch changes it to use XOBNEWVEC.

Also, the type_unit_unshareable::num_symtabs member is assigned but
never read.  So, this removes it.

gdb/ChangeLog
2020-02-24  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.h (struct type_unit_unshareable) <num_symtabs>:
	Remove.
	* dwarf2/read.c (dwarf2_cu::setup_type_unit_groups): Use
	XOBNEWVEC.