[graphite] Remove mostly extraneous RPDesc label tracing

This avoids generating a fresh semi-long string for each DrawPass's
trace event and having to copy said string with TRACE_STR_COPY.
Hopefully it reduces the claimed CPU time spent in Recorder::snap().

Change-Id: I3ac46903656f6f5e34dec118a92a66792d72143c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1283357
Commit-Queue: Noelle Scobie <nscobie@google.com>
Auto-Submit: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Noelle Scobie <nscobie@google.com>