[AOTI][refactor] Remove is_legacy_abi_kernel and abi_compatible_kernel (#121523)
Summary: is_legacy_abi_kernel was used for _scaled_dot_product_flash_attention fallback. It is only needed for C shim kernel name matching now, and the name matching is done with a direct string comparison. Also consolidate the fallback cpp kernel naming logic in CppWrapperCpu.
Differential Revision: [D54727789](https://our.internmc.facebook.com/intern/diff/D54727789)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121523
Approved by: https://github.com/chenyang78
