[docs] Remove mention of -fslp-vectorize-aggressive

[vsk]

Summary

I think it's time to remove mention of this flag, since it looks like it's been deprecated since cfe/r306786.

docs/Vectorizers.rst

 ------
 
 The SLP Vectorizer is enabled by default, but it can be disabled
 through clang using the command line flag:
 
 .. code-block:: console
 
    $ clang -fno-slp-vectorize file.c
-
-LLVM has a second basic block vectorization phase
-which is more compile-time intensive (The BB vectorizer). This optimization
-can be enabled through clang using the command line flag:
-
-.. code-block:: console
-
-   $ clang -fslp-vectorize-aggressive file.c
-