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>

fs.c

@@ -84,7 +84,6 @@ bfree(int dev, uint b)
   struct buf *bp;
   int bi, m;
 
-  readsb(dev, &sb);
   bp = bread(dev, BBLOCK(b, sb));
   bi = b % BPB;
   m = 1 << (bi % 8);