sysent.mk: remove CAPABILITIES_CONF support

[brooks]

Summary

The last consumer was removed with a switch to generating freebsd32's
syscall files from sys/kern/syscalls.master in commit be67ea40c5a0.

This followed prior work to use CAPENABLED flags in syscalls.master
instead of capabilities.conf in commit df501bac6939.

[brooks]

This was committed in d6bc956e143b194c221a9e697cd45a8a62028e86 and I forgot to add the Differential Revision line.