Remove unnecessary mul and assignment in gradient shaders.

Remove unnecessary mul and assignment in gradient shaders.

R=robertphillips@google.com
Committed: https://code.google.com/p/skia/source/detail?r=6201

[bsalomon, 2012-10-30 15:38:09]

For some reason we wound up doing the modulation with the incoming color twice
and generating code that looks like this:

color0 *= vColor;
color0 = vec4(vColor * texture2D(uSampler0, coord));

This removes the first line.

[robertphillips, 2012-10-30 15:46:48]

LGTM.