Remove references to Index8

BitmapFactory will already not decode to Index8, so stop acting like it
will. Remove colorTypeForScaledOutput, which was only meaningful for
kIndex_8.

Will allow re-landing
https://skia.googlesource.com/skia/+/742a3e298fda669006147e4a305bab8452369b1f

Test: None. This does not change behavior.

Bug: 62483967
Bug: skbug.com/6828

Change-Id: I8d8182436a5c9d9858934183a53eda631f9270d5