(PA-5654) Remove warnflags from rbconfig.rb on AIX

[AriaXLi]

The validate_vendored_ruby.rb test is failing on AIX when trying to compile native extensions for the sqlite3 gem. The gcc version used in this test (gcc 5.2.0) does not recognize the warnflags used in rbconfig.rb. This is because a newer gcc (gcc 10) was used when when creating rbconfig.rb.

This commit removes the warnflags in rbconfig.rb on AIX.

[AriaXLi]

I tested my changes locally to make sure the test passes now using these commands:

$ bundle exec beaker-hostgenerator redhat7-64m-aix72-POWERfa%7Bpackaging_platform=aix-7.2-power%7D --hypervisor abs --templates-only --disable-default-role
$ bundle exec rake ci:test:setup HOSTS=hosts.yaml SHA=7de9105be1c2823ca833cb28face1a615cdddc07
$ bundle exec beaker exec tests/validate_vendored_ruby.rb
...
tests/validate_vendored_ruby.rb passed in 193.82 seconds
      Test Suite: tests @ 2023-08-14 16:54:47 -0700

      - Host Configuration Summary -

              - Test Case Summary for suite 'tests' -
       Total Suite Time: 193.82 seconds
      Average Test Time: 193.82 seconds
              Attempted: 1
                 Passed: 1
                 Failed: 0
                Errored: 0
                Skipped: 0
                Pending: 0
                  Total: 1