Remove Giant asserts. Update comment.

Owning Giant in the init/uninit is accidental due to the moment where
VFS modules initialization is performed, and is not enforced by the
VFS interface. The Giant lock does not prevent a parallel execution
of the code, it is VFS which implements the proper protocol.

Approved by: des (pseudofs maintainer)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week