Remove -O[0123s] gcc optimization flags for gcc pass 4 speedups again.

Remove -O[0123s] gcc optimization flags for gcc pass 4 speedups again.

We cannot guarantee that (un)optimized code compiles and/or generates
fully functional code, so don't tempt the user to try it out.

* session.h (struct systemtap_session): Remove gcc_flags string field.
* hash.cxx (find_script_hash): Don't add gcc_flags.
* main.cxx (usage): Remove -O[0123s] documentation.
(main): Don't use gcc_flags.
* buildrun.cxx (compile_pass): Don't add gcc_flags to EXTRA_CFLAGS.
* stap1.in: Remove -O[0123s] documentation.
* testsuite/systemtap.base/cache.exp: Remove tests for -O[0123s].