gdb: dwarf2/read.c: remove unused objfile parameters/variables

This is a simple cleanup.  These functions used to use the objfile's
obstack for allocation in the hash tables, but they don't anymore.
Remove the unnecessary objfile parameters, which in turn allows removing
some local variables.

gdb/ChangeLog:

	* dwarf2/read.c (allocate_signatured_type_table,
	allocate_dwo_unit_table, allocate_type_unit_groups_table,
	allocate_dwo_file_hash_table, allocate_dwp_loaded_cutus_table):
	Remove objfile parameter, update all callers.