Revert of Add RGB565 Texture readback support in gl_helper

Revert of Add RGB565 Texture readback support in gl_helper (https://codereview.chromium.org/88033002/)

Reason for revert:
content_gl_tests failing on multiple gpu bots on the chromium.webkit buildbot.

Example: http://chromegw.corp.google.com/i/chromium.webkit/builders/GPU%20Mac10.7/builds/19123

Original issue's description:
> Add RGB565 Texture readback support in gl_helper
> 
> Plan is to add a RGB565 readback and copy conversion for RGBA8 to RGB565 in GLHelper.
> 
> BUG=323150
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=245431

TBR=dtrainor@chromium.org,sievers@chromium.org,hubbe@chromium.org,aelias@chromium.org,piman@chromium.org,siva.gunturi@samsung.com
NOTREECHECKS=true
NOTRY=true
BUG=323150
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=245477

[Mads Ager (chromium), 2014-01-17 10:00:28]

Created Revert of Add RGB565 Texture readback support in gl_helper

[commit-bot: I haz the power, 2014-01-17 10:03:26]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/ager@chromium.org/137783022/1

[commit-bot: I haz the power, 2014-01-17 10:09:40]

Change committed as 245477