NativeThreadProtocol: delete dead code

These functions were unused as everyone just went straight for the
direct operations on the register context. In fact, the
Save/RestoreAllRegisters actually appear to be wrong (inverted). Thanks
to Tatyana for pointing this out.

These functions are not very useful now that we can guarantee that each
thread always contains a valid register context, so I just delete them.

llvm-svn: 328770