thumb: Remove a brittle, ugly and unused arm->thumb conversion
thumb: Remove a brittle, ugly and unused arm->thumb conversion

The relevant code that this conversion handled was removed in
c26a9ecaa21f4.

Change-Id: Iee40f95134e609c291c7c4e06bc50dcb895bc5e3

webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx