isdn: Remove unused get_Bprotocol4id()

get_Bprotocol4id() was added in 2008 in
commit 1b2b03f8 ("Add mISDN core files")
but hasn't been used.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Link: https://patch.msgid.link/20241211005802.258279-1-linux@treblig.org


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