mISDN: remove unused struct 'bf_ctx'

'bf_ctx' appears unused since the original
commit 960366cf ("Add mISDN DSP").

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Link: https://lore.kernel.org/r/20240523155922.67329-1-linux@treblig.org


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