logfs: remove from tree

Logfs was introduced to the kernel in 2009, and hasn't seen any non
drive-by changes since 2012, while having lots of unsolved issues
including the complete lack of error handling, with more and more
issues popping up without any fixes.

The logfs.org domain has been bouncing from a mail, and the maintainer
on the non-logfs.org domain hasn't repsonded to past queries either.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>