grep: fix/remove references to -P

-P in gnugrepland means PCRE, which we do not support. We may eventually
support it if onigmo ends up getting imported as a more performant regex
implementation, and we can re-add it properly in these places (and more)
when that time comes.

The optstr change is a functional nop; the case was not explicitly handled,
thus ending in usage() anyways.

Reported by: Vladimir Misev (via twitter)