tty: caif: removed unused function debugfs_tx()

Remove debugfs_tx() which was added when the caif driver was added in
commit 9b27105b ("net-caif-driver: add CAIF serial driver (ldisc)")
but it has never been used.

Flagged by LLVM 19.1.7 W=1 builds.

Signed-off-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250320-caif-debugfs-tx-v1-1-be5654770088@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>