Remove static asserts for GrSLType and GrVertexAttribType enum values.

Replaces with switch statements that will produce a compiler warning if a type is added and helper functions are not updated.

This also removes several unused helper functions for these enum types.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4486

Change-Id: Iee6d8e5095dc4ba2488a8928bf60f8e903cd3320
Reviewed-on: https://skia-review.googlesource.com/4486
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>