[utils] remove ability to generate llc check lines from update_test_checks.py

The dream of a unified check-line auto-generator for all phases of compilation is dead.
The llc script has already diverged to be better at its goal, so having 2 scripts that
do almost the same thing just causes confusion. Now, this script will only work with 
opt to produce check lines for IR transforms.

llvm-svn: 305208