tty/ldsem: Remove unused ldsem_down_write_trylock

ldsem_down_write_trylock() was added in 2013 by
commit 4898e640 ("tty: Add timed, writer-prioritized rw semaphore")
but hasn't been used.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250122012559.441006-1-linux@treblig.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>