readme: remove outdated SIMD info
readme: remove outdated SIMD info

Looks like the upstream brew Formula [0][] now has SIMD support, so
remove the extraneous info now that the custom tap is no longer needed
[1][].

[0]: https://github.com/Homebrew/homebrew-core/blob/master/Formula/ripgrep.rb
[1]: https://github.com/BurntSushi/ripgrep/commit/f3083e4574ad20881de66fdeb66d671f1cbdfda4

PR #1431 
BurntSushi/ripgrep,BurntSushi/ripgrep,BurntSushi/ripgrep,BurntSushi/ripgrep