Remove GrGLSLSampler type and subclasses

GrGLSLUniformManager and GrGLSLProgramDataManager subclasses handle sampler variation across GPU backends.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5002

Change-Id: I968b006bab978c2fa209d1f7eda710c298d1212f
Reviewed-on: https://skia-review.googlesource.com/5002
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
27 files changed