Fcntl: remove pointless S_ISENFMT function

[mauke]

S_ISENFMT($mode) has no equivalent C macro. It was implemented (in XS) as the equivalent of ($mode & S_IFMT) == S_ENFMT, which is always false because S_ENFMT is not a file type and thus not covered by the S_IFMT mask.

(See commits ca6e1c2, 0e6f150, 0f68039, fb59364, 87eca6e for the history of this function, which has always been broken in various ways ever since it was added in 2000.)

Fixes #22190.