Revert "Let client pass in full extension to GrVkBackendContext."

This reverts commit 45c9dab4c3ec43cedb28d1b8c08e166fe0c2e767.

Reason for revert: fucshia uses GrVkBackendContext. Need to revert earlier changes

Original change's description:
> Let client pass in full extension to GrVkBackendContext.
> 
> Bug: skia:
> Change-Id: I772ab4ccbca0f4f7e7d429d6c421b07d97f0606f
> Reviewed-on: https://skia-review.googlesource.com/131880
> Reviewed-by: Jim Van Verth <jvanverth@google.com>
> Commit-Queue: Greg Daniel <egdaniel@google.com>

TBR=egdaniel@google.com,jvanverth@google.com,bsalomon@google.com

Change-Id: I1a765ff406c83cb234c3614b804fbed677d5a382
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/137901
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
13 files changed