fortran: Delete redundant missing_arg_type field

Now that we can get information about an actual arg's associated
dummy using the associated_dummy attribute, the field missing_arg_type
contains redundant information.
This removes it.

gcc/fortran/ChangeLog:
	* gfortran.h (gfc_actual_arglist::missing_arg_type): Remove.
	* interface.c (gfc_compare_actual_formal): Remove
	missing_arg_type initialization.
	* intrinsic.c (sort_actual): Ditto.
	* trans-expr.c (gfc_conv_procedure_call): Use associated_dummy
	and gfc_dummy_arg_get_typespec to get the dummy argument type.