gdb: Update comments that reference VEC or vec.h

gdb: Update comments that reference VEC or vec.h

With the removal of the old VEC mechanism from the code base, update
comments that still make reference to VECs.  There should be no user
visible changes after this commit.

gdb/ChangeLog:

	* linespec.c (decode_digits_ordinary): Update comment.
	* make-target-delegates: No longer need to handle VEC case.
	* memrange.c (normalize_mem_ranges): Update comment.
	* namespace.c (add_using_directive): Update comment.
	* objc-lang.c (uniquify_strings): Update comment.
	* ppc-linux-nat.c (struct thread_points): Update comment.
	* probe.h (find_probes_in_objfile): Update comment.
	* target.h (enum flash_preserve_mode): Update comment.
	* varobj.c (varobj_restrict_range): Update comment.
	* varobj.h (varobj_list_children): Update comment.

Change-Id: Iefd2e903705c3e79cd13b43395c7a1c167f9a088