[Driver] Delete -mimplicit-it=

This is a GNU as and Clang cc1as option, not a GCC option.
Users should specify `-Wa,-mimplicit-it=` instead.

Note: mixing the -m option and the -Wa, option doesn't work
`-Wa,-mimplicit-it=never -mimplicit-it=always` =>
`clang (LLVM option parsing): for the --arm-implicit-it option: may only occur zero or one times!`

Reviewed By: nickdesaulniers, raj.khem

Differential Revision: https://reviews.llvm.org/D102568