kernel-yocto: remove branch existence checking in do_validate_branches

kernel-yocto: remove branch existence checking in do_validate_branches

Now that the fetcher will enforce branch existence, we no longer need to
confirm that a branch exists, and that it was the branch requested to
be built.

We know the branch exists and we'll confirm that the specified SRCREV
is going to be built after we've patched the tree.

(From OE-Core rev: 93a7c7bd8e860e621af7174ef10d571b0d8622b2)

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