[UpdateTestChecks] Remove outdated help text

Manually modifying the result of update_test_checks.py is discouraged,
we prefer unmodified check lines where possible. The output is also
considered authoritative nowadays, at least for tests targeting core
middle-end components, where not using it is an automatic review
rejection.

Differential Revision: https://reviews.llvm.org/D129259

llvm/utils/update_test_checks.py

@@ -22,11 +22,6 @@
 7. Re-run this script on affected regression tests.
 8. Check the diffs to ensure the script has done something reasonable.
 9. Submit a patch including the regression test diffs for review.
-
-A common pattern is to have the script insert complete checking of every
-instruction. Then, edit it down to only check the relevant instructions.
-The script is designed to make adding checks to a test case fast, it is *not*
-designed to be authoratitive about what constitutes a good test!
 """
 
 from __future__ import print_function