Move one function off of branchman.c and remove source file.

Move one function off of branchman.c and remove source file.

After branch-management support was added, branchman.c became obsolete: it
used to hold the old ioctl-based branch-management commands, but now we do
those commands via remount (in super.c).  So move the only remaining small
query-file ioctl from branchman.c to commonfops.c, close to unionfs_ioctl;
and remove any leftover extern's which referred to old code in branchman.c