remount: Remove unused option -C/--clean_scratch_files

The option was added for debugging purposes in case the "test argv[0] ==
clean_scratch_files" method breaks.
Now it has no user at all and no reason to maintain it, so just remove it.

Also don't use MyLogger() when running as clean_scratch_files, as
clean_scratch_files don't have stdout/stderr.

Bug: 241179247
Test: Presubmit
Change-Id: I2d8069f59fe6b85fc84ab07bb2df6efb39d6ecaa