skpbench: remove --path flag

Replaces the confusing --path flag with a positional argument that
gives the path to the skpbench binary.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4635

Change-Id: I2b5eeca61ec85e7fe45fd3370625eddf34a2fd0e
Reviewed-on: https://skia-review.googlesource.com/4635
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
1 file changed