voice_extn: remove redundant check of mode when stop voice use case

voice use case is still active when mode switched from MODE_IN_CALL
to MODE_IN_COMMUNICATION, while the output used for driving call is
NULL.

This could possibly makes segment fault in mediaserver when call
is activated before the output updated to valid value.

Change-Id: I070dd5e7a1c6d2883ea611588737b1f5c577b275
CRs-Fixed: 953010