[RISCV][NFC] Remove redundant test cases. (#91324)

PR #89727 added the two test cases to verify `.option arch` should only
work when having -menable-experimental-extensions. And the test idea
could be splitted to
1. When having menable-experimental-extensions, clang passes
+experimental.
2. `.option arch` only enabled when +experimental enabled. 

And we already had the two kind of tests.

Author: yetingk