kernel-yocto: remove KBRANCH_DEFAULT

kernel-yocto: remove KBRANCH_DEFAULT

KBRANCH_DEFAULT was introduced as a way to trigger the enforced build
of a particular branch of the tree. With the fetcher now enforcing
SRCREVs existing on a branch, we can simply validate that the SRCREV
is reachable from the final branch and no longer care about enforcing
a given branch.

(From OE-Core rev: fbacbb0ca79cdae33803fdd3158671488b9bbcbe)

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>