nfscl: Delete unneeded redundant MODULE_DEPEND() calls

There are two module declarations in the nfscl.ko module for "nfscl"
and "nfs". Both of these declarations had MODULE_DEPEND() calls.
This patch deletes the MODULE_DEPEND() calls for "nfs" to avoid
confusion with respect to what modules this module is dependent upon.

The patch also adds comments explaining why there are two module
declarations within the module.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D30102