Avoid rereading the superblock in bfree

Avoid rereading the superblock in bfree

The superblock is read when the first process returns to
userspace (via the call to `iinit` from the first invocation of
`forkret`, when `first` is still set to 1) and cached in a
global; it is treated as immutable from that point on.  There is
no reason to re-read it in bfree.

Removing this call seems to have simply been missed in 8320d61b
when this general area was last touched.  Remove it now to avoid
potential confusion.

Signed-off-by: Dan Cross <cross@gajendra.net>