vfs: remove dead fifoop VOP_KQFILTER implementations

These began to become obsolete in d6d64f0f2c26 (r137739) and the deal
was later sealed in 003e18aef4c4 (r137801) when vfs.fifofs.fops was
dropped and vop-bypass for pipes became mandatory.

PR: 225934
Suggested by: markj
Reviewe by: kib, markj
Differential Revision: https://reviews.freebsd.org/D32270