remove trim_fs method from Documentation/filesystems/Locking

The ->trim_fs has been removed meanwhile, so remove it from the documentation
as well.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>