Remove GR_GL_USE_NEW_SHADER_SOURCE_SIGNATURE flag.

Enough's enough. It's been a long time since the standard was updated.

Change-Id: I49ae08ee49f06cb8403c9cb72a811edd35b25575
Reviewed-on: https://skia-review.googlesource.com/c/178930
Auto-Submit: Brian Salomon <bsalomon@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
4 files changed