Resubmit "[MLIR] Remove unused config attributes from lit.site.cfg.py"

This resubmits commit 0816b629, reverted in commit 328bbab5, but without removing the config.target_triple.

Lit checks UNSUPPORTED tags in the input against the config.target_triple (https://llvm.org/docs/TestingGuide.html#constraining-test-execution).

The original commit made the following bots start failing, because unsupported tests were no longer skipped:
- s390x: https://lab.llvm.org/buildbot/#/builders/199/builds/9247
- Windows: https://lab.llvm.org/buildbot/#/builders/13/builds/25321
- Sanitizer: https://lab.llvm.org/buildbot/#/builders/5/builds/27187