Remove handling for WEBP_HINT_GRAPH

Remove handling for WEBP_HINT_GRAPH w.r.t use_palette flag.

The WEBP_HINT_GRAPH is now used at one place, to set the initial size of the
Bit Writer as bpp for photo images are generally larger than the graphical
images.

Change-Id: I1b9c4436c85a8f69da74c0dbcd292397323f2696