remove mask load() and store()

They appear to be slower than the generic load() and store() now.

    [blendmode_mask_Hue]    14.7ms  @0  15.6ms  @95 39.6ms  @100
    [blendmode_rect_Hue]    31.5ms  @0  37.6ms  @95 39.5ms  @100
    ~~>
    [blendmode_mask_Hue]    14.7ms  @0  15.2ms  @95 39.5ms  @100
    [blendmode_rect_Hue]    30.5ms  @0  32.6ms  @95 37.8ms  @100

Change-Id: I674b75087b8139debead71f3016631bcb0cb0047
Reviewed-on: https://skia-review.googlesource.com/33800
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed