This removes the dead code for the IBM tile cache APIs. These have been
unused for as far back as I could manage to search[1].
While investigating moving TIFF to OIIO as part of T101413, I found that
the imb_loadtiletiff call was orphaned and thought it best to clean it
up before moving forward.
The cache was introduced here for 2.53: https://developer.blender.org/rB008863daec1249d1f17bc69e1105e336db690d63
But I can't find the point where it was actually used in master? Perhaps it was only used for one of the open-movie branches at the time?