Remove EXT_LANG_BT_COMPLETED

Remove EXT_LANG_BT_COMPLETED

While looking at the frame filter code, I noticed that
EXT_LANG_BT_COMPLETED is not really needed.  Semantically there is no
difference between the "completed" and "ok" results.  So, this patch
removes this constant.

gdb/ChangeLog
2018-03-26  Tom Tromey  <tom@tromey.com>

	* python/py-framefilter.c (py_print_frame): Return
	EXT_LANG_BT_OK.
	(gdbpy_apply_frame_filter): Update comment.
	* extension.h (enum ext_lang_bt_status) <EXT_LANG_BT_COMPLETED>:
	Remove.
	<EXT_LANG_BT_NO_FILTERS>: Change value.