Remove an Outdated Description in INSTALL.md (#2391)

Summary:
According to [`CMakeLists.txt`](442d9f4a2d/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and confirmed that (current `faiss` treats `sse4` as same as `generic`), so this PR removes the description of this outdated option from `INSTALL.md` .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

INSTALL.md

@@ -101,8 +101,8 @@ Several options can be passed to CMake, among which:
   - `-DCMAKE_BUILD_TYPE=Release` in order to enable generic compiler
   optimization options (enables `-O3` on gcc for instance),
   - `-DFAISS_OPT_LEVEL=avx2` in order to enable the required compiler flags to
-  generate code using optimized SIMD instructions (possible values are `generic`,
-  `sse4`, and `avx2`, by increasing order of optimization),
+  generate code using optimized SIMD instructions (possible values are `generic`
+  and `avx2`, by increasing order of optimization),
 - BLAS-related options:
   - `-DBLA_VENDOR=Intel10_64_dyn -DMKL_LIBRARIES=/path/to/mkl/libs` to use the
   Intel MKL BLAS implementation, which is significantly faster than OpenBLAS