fs/pipe: remove buggy and unused 'helper' function

While looking for incorrect users of the pipe head/tail fields (see
commit c27c66af: "fs/pipe: Fix pipe_occupancy() with 16-bit
indexes"), I found a bug in pipe_discard_from() that looked entirely
broken.

However, the fix is trivial: this buggy function isn't actually called
by anything, so let's just remove it ASAP.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>