Remove is_a_field_of_this from ada_lookup_symbol

Remove is_a_field_of_this from ada_lookup_symbol

All callers of ada_lookup_symbol pass NULL for the
"is_a_field_of_this" parameter, so remove it.

gdb/ChangeLog
2019-07-01  Tom Tromey  <tromey@adacore.com>

	* ada-exp.y (find_primitive_type): Update.
	* ada-lang.h (ada_lookup_symbol): Update.
	* ada-lang.c (ada_lookup_symbol): Remove "is_a_field_of_this"
	parameter.
	(ada_lookup_encoded_symbol, ada_lookup_symbol_nonlocal): Update.