sysfs: introduce [__]sysfs_remove()

Given a sysfs_dirent, there is no reason to have multiple versions of
removal functions.  A function which removes the specified
sysfs_dirent and its descendants is enough.

This patch intorduces [__}sysfs_remove() which replaces all internal
variations of removal functions.  This will be the only removal
function in the planned new sysfs_dirent based interface.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>