Remove unused virtuals from ABISysV_ppc*

Summary:
The StackUsesFrames and FunctionCallsChangeCFA virtual functions
aren't used anywhere and aren't overridden by anything.

They were introduced when the ABISysV_ppc* code was added and weren't
used at the time. The review for the commit that added them can be
found at http://reviews.llvm.org/D5988

The commit comment notes that backtraces don't yet work:

    Backtraces don't work. This is due to PowerPC ABI using a
    backchain pointer in memory, instead of a dedicated frame
    pointer register for the backchain.

So there is a possibility these were added with the intent of using
them in the future.

Reviewers: clayborg, jingham, jhibbits, emaste

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D13506

llvm-svn: 249563