fs/jfs: Remove dead code

Since the JFS code was first added to Linux, there has been code hidden
in ifdefs  for some potential future features such as defragmentation
and supporting block sizes other than 4KB. There has been no ongoing
development on JFS for many years, so it's past time to remove this dead
code from the source.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>