[NFC][update_llc_test_checks] Remove the redundant SCRUB_LOOP_COMMENT_RE in asm.py

Summary:
In the patch: https://reviews.llvm.org/D42654
De-duplicate utils/update_{llc_,}test_checks.py, Some common part has
been move to common.py. The SCRUB_LOOP_COMMENT_RE has been moved to
common.py, but forgetting to remove from asm.py.
This patch is to remove the redundant SCRUB_LOOP_COMMENT_RE in asm.py
and use common.SCRUB_LOOP_COMMENT_RE.