gdb: remove TYPE_DYN_PROP_LIST macro

gdb: remove TYPE_DYN_PROP_LIST macro

Remove this macro, which abstracts how to obtain the dyn_prop_list of a
given type.  We could replace it with a method on `struct type`, but I
don't think it's needed, as the only code that accesses the dynamic prop
list directly is internal gdbtypes.c code (that can be seen as code
internal to `struct type`).  So it can just refer to the field directly.

gdb/ChangeLog:

	* gdbtypes.h (TYPE_DYN_PROP_LIST): Remove.  Update all users
	access thistype->main_type->dyn_prop_list directly.