vfs: revert the overzealous assert added in r357285 to vgone

The intent was to make it more likely to catch filesystems with custom
need_inactive routines which fail to call vn_need_pageq_flush (or do an
equivalent).

One immediate case which is missed is vgone from called by inactive itself.

A better assertion may land later. The routine is not added to vputx because
it is of no use to tmpfs et al.

Reported by: syzbot+5f697ec11f89b60941db@syzkaller.appspotmail.com