[MemorySSA] remove cruft; NFC.

The caching walker used to hold its own caches, which made its `reset()`
function meaningful. Since caching has been moved out of it, there's no
reason to continue to have these cache-related methods.

Similarly, the EXPENSIVE_CHECKS block that's getting removed used to
rerun the query with caching disabled. Since that's how we always do
queries now, it's redundant.

llvm-svn: 329638