Light cleanup of GrTextBlob cache

* Remove unused methods
* GrTextBlobCache::makeBlob -> GrTextBlob::Make
* Fix up comments
* Move impl to .cpp

Change-Id: Ife4de04e709d3a6d212486f631bc67561957cdd6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/291198
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
3 files changed