Revert "[clang-repl] Ensure clang-repl accepts all C keywords supported in all language models (#142749) (#142933)
This broke CI on platforms such as PPC64LE and AIX due to _Float16 not being supported.
We will reintroduce the changes later with proper platform guards and tests.
This reverts commit 7ca7bcb7d8dcf26fc0281697fe47aa6cdb3884c0.