rust: Drop TARGET_VENDOR export
This was used by crossbeam_atomic.patch, but that patch was removed as
part of the 1.69.0 upgrade.
(From OE-Core rev: 3753997d6ec19e8cd082f9a602ebe4a7c1308c36)
Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>