Remove the redundant manual numbering. Also rename
AllGraphicsShadersReadWrite to AllGraphicsShadersWrite for consistency.

Refactoring change only.

Bug: angleproject:4959
Change-Id: I2e7ca00993f192897bbf88f4bdc3f1610bcb345f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2354279
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Charlie Lao <cclao@google.com>