geom_ctl.c: remove stale header files

geom_ctl.c: remove stale header files

- Remove "opt_geom.h", no kernel options are used.

- Remove <sys/sysctl.h>, no sysctl functionality is used here.

- Remove <sys/bio.h>, requirements for bio moved out in r112534.

- Remove <sys/lock.h> and <sys/mutex.h>, last used by DROP_GIANT() and
  PICKUP_GIANT(), which were removed in r115624.

- Remove <sys/disk.h> and <sys/kernel.h>, not used.

Reviewed by: phk, kevans (mentor)
Approved by: phk, kevans (mentor)
Differential Revision: https://reviews.freebsd.org/D26805