Remove an Outdated Description in INSTALL.md (#2391)
Remove an Outdated Description in INSTALL.md (#2391)

Summary:
According to [`CMakeLists.txt`](https://github.com/facebookresearch/faiss/blob/442d9f4a2d43e2ada6423e8e4e3414131dea849d/CMakeLists.txt#L20), current `faiss` doesn't recognize `sse4` as `FAISS_OPT_LEVEL` .
I've read `CMakeLists.txt`s and confirmed that (current `faiss` treats `sse4` as same as `generic`), so this PR removes the description of this outdated option from `INSTALL.md` .

This PR contains only document update, so this doesn't affect the software behavior.

Pull Request resolved: https://github.com/facebookresearch/faiss/pull/2391

Reviewed By: alexanderguzhva

Differential Revision: D39167022

Pulled By: mdouze

fbshipit-source-id: ff36fc5167c4d2e8d16206061624a8ba2890b4b7

facebookresearch/faiss,facebookresearch/faiss,facebookresearch/faiss,facebookresearch/faiss