remove purge more interface from GrTextBlobCache

Change-Id: Ice1b7593ebf9cf38191c180eb56183ffc6167f0b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/299293
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Herb Derby <herb@google.com>
6 files changed