remove now defunct get_paint_offset

This used to be used when we wanted to know where the paint was, so we could perform peep-hole optimizations.

We no longer do that on this (older) picture format, so now this function only serves to clog up the file with asserts.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1557123002

Review URL: https://codereview.chromium.org/1557123002
1 file changed