Remove references to PIR in the repo.

[coke]

Description of issue

References to PIR are no longer relevant, there is no version for years now that has this functionality, it predates the v6.c release of the language.

[petdance]

That sounds like I should remove pir from the list of extensions that ack recognizes as --parrot.

[petdance]

beyondgrep/ack3#81

[coke]

no. Parrot still uses pir. Perl 6 no longer uses parrot.