Simplify dwarf2-frame.c:read_addr_from_reg.

Simplify dwarf2-frame.c:read_addr_from_reg.

Since 'struct dwarf_expr_context_funcs::read_addr_from_reg' is now
only used for addresses, we can make it use unpack_pointer.  And since
we now have 'struct dwarf_expr_context_funcs'::get_reg_value, there's
no need for speculation about using values here.

Tested on x86_64 Fedora 17.

gdb/
2013-11-18  Pedro Alves  <palves@redhat.com>

	* dwarf2-frame.c (read_addr_from_reg): Remove stale comment and
	use unpack_pointer.