remove unused fs/befs/attribute.c

If anyone needs a fully-functional befs driver, the easiest route to
that would probably be getting Haiku's befs driver to compile in
userland as a FUSE fs.

At any rate, attribute.c can go. It is easy enough to add back in if
anyone ever wants to do the (relativly minor) refactoring nessisary to
get it working.

Signed-off-by: Will Dyson <will.dyson@gmail.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>