feature-arm-thumb.inc: drop legacy _thumb and _thumb-interwork over-rides

feature-arm-thumb.inc: drop legacy _thumb and _thumb-interwork over-rides

Bitbake over-rides for _thumb and _thumb-interwork are undocumented
and are not used anywhere in oe-core or meta-oe. The logic setting up
the thumb-interwork over-ride even seems to be reversed and nobody
noticed, so it seems safe to assume that these over-rides are not
used.

(From OE-Core rev: 351443d71eb246a946b41f12b54d57b36fe1574e)

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>