sound: oss: dmasound: remove software_input_volume declaration

expand_read_bal has been removed since commit fc37449f7959 ("The
next round of scheduled OSS code removal").

software_input_volume has been removed since
commit 0a1b42db4bf9 ("sound: sound/oss/dmasound/: cleanups").

so remove the declare for them from header file.

Signed-off-by: Gaosheng Cui <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

sound/oss/dmasound/dmasound.h

···
 #define SW_INPUT_VOLUME_SCALE	4
 #define SW_INPUT_VOLUME_DEFAULT	(128 / SW_INPUT_VOLUME_SCALE)
 
-extern int expand_read_bal;	/* Balance factor for reading */
-extern uint software_input_volume; /* software implemented recording volume! */
-
 #endif /* _dmasound_h_ */