Don't use gdb_py_long_from_longest

Change the Python layer to avoid gdb_py_long_from_longest, and remove
the defines.

gdb/ChangeLog
2020-09-15  Tom Tromey  <tromey@adacore.com>

	* python/python-internal.h (gdb_py_long_from_longest): Remove
	defines.
	* python/py-value.c (valpy_long): Use gdb_py_object_from_longest.
	* python/py-type.c (convert_field, typy_get_sizeof): Use
	gdb_py_object_from_longest.
	* python/py-record-btrace.c (btpy_list_index): Use
	gdb_py_object_from_longest.