benchsuite: remove -a flag from grep
benchsuite: remove -a flag from grep

It's not quite clear why I added this originally. ripgrep doesn't have
its `-a` flag enabled. It's possible I tricked myself into adding it
because ripgrep's binary detection has evolved to be more like GNU
grep's nowadays.

In any case, using `-a` on data that is non-binary can only improve
performance because it removes the overhead for checking whether the
data is binary or not. So this was giving an artificial boost to GNU
grep.

BurntSushi/ripgrep,BurntSushi/ripgrep,BurntSushi/ripgrep,BurntSushi/ripgrep