mithrax: Remove Burnside Bridge config
mithrax: Remove Burnside Bridge config

mithrax does not use Burnside Bridge. Reduce image size and avoid linker
errors when building without TBT and USB4 functions, which mithrax also
does not use.

BUG=b:244467636
TEST=make BOARD=mithrax
BRANCH=none

Signed-off-by: Abe Levkoy <2e1ad7039298f93262cf06bbb457275fb3e2ecb5@chromium.org>
Change-Id: I6945656f8a63412420d729f08ecace66fec2be18
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3887963
Reviewed-by: Boris Mittelberg <46897f4fb5cad4b1d5146ad1fd460d7f4825bbf7@google.com>
Reviewed-by: caveh jalali <08e24a66aefda7a55ca7d73f8903e0fb16eec763@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec