binderfs: remove separate device_initcall()

binderfs should not have a separate device_initcall(). When a kernel is
compiled with CONFIG_ANDROID_BINDERFS register the filesystem alongside
CONFIG_ANDROID_IPC. This use-case is especially sensible when users specify
CONFIG_ANDROID_IPC=y, CONFIG_ANDROID_BINDERFS=y and
ANDROID_BINDER_DEVICES="".
When CONFIG_ANDROID_BINDERFS=n then this always succeeds so there's no
regression potential for legacy workloads.

Signed-off-by: Christian Brauner <christian@brauner.io>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>