Remove unnecessary #includes from the kqueue(2) man page.

Now that <sys/event.h> can be included on its own, adjust the manual
page accordingly. Remove both unnecessary #include statements from the
synopsis and the example code.

While there, also add a note to the BUGS section to mention that
previous versions of this header file still depend on <sys/types.h>.

Reviewed by: ngie, vangyzen
Differential Revision: https://reviews.freebsd.org/D9605