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.

This is a cherrypick from 3abdb1ca658c84ba122ef04229abe8c8a2f2ed03
in libvpx.

Change-Id: I70ae2856b324f1c4a54807dabfadfcf5645d7adb

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom