remove KMDB branch debugging support
KMDB has vestigial support for two features, ::step branch and ::branches. The former allows you to step to the next branch point, while the latter reports the "last few" branches, as recorded in MSRs such as the LBR.
However, this was never properly implemented for 64-bit, thus is effectively non-existent on any reasonably modern system. While it could be resurrected, fixing this would complicate KPTI implementation. As it's already broken, it's hard to argue that it will be missed; we'll just remove all the relevant code.