Clean up some comments in minsyms.c

Clean up some comments in minsyms.c

Philippe pointed out that some comments in minsyms.c still referred to
obstack allocation.  This patch fixes these up.

In most cases here, my view is that the comments were more misleading
than helpful.  So, I've generally removed text.

gdb/ChangeLog
2019-03-25  Tom Tromey  <tromey@adacore.com>

	* minsyms.c (BUNCH_SIZE): Update comment.
	(~minimal_symbol_reader): Remove old comment.
	(compact_minimal_symbols): Update comment.
	(minimal_symbol_reader::install): Remove old comment.  Update
	other comments.