Cleanup unused cowsay files (related to legacy Perl binary version)

Summary:
Premising that Remarkup supports a fully-featured cowsay generator (!)... since 2015 this awesome
thing no longer requires a binary cowsay command, since everything was rewritten in the core:

c705c8011e964945a6f73ae78087e188de0bd71f

So, some files from the legacy cowsay version in Perl can be safely removed.

We still thank the original author Tony Monroe for the original version in Perl (dated 2000),
and Evan Priestley for the re-implementation in PHP.

Probably nobody knows about this feature. This may change in the future. See T15401.

Closes T15418

Test Plan:
Try some awesome cows to verify that everything still works. For instance:

cowsay {{{
 hello
}}}

cowsay (think=yes,eyes=XX,tongue=^^) {{{
 hello
}}}

cowsay (cow=stegosaurus) {{{
 hello
}}}

If nothing explodes, yuppie! Cowsay still works.

Credits to the original version in Perl remained in place.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: avivey, speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15418

Differential Revision: https://we.phorge.it/D25242