Drivers: hv: vmbus: Remove unused code to check for subchannels

Drivers: hv: vmbus: Remove unused code to check for subchannels

The last caller of vmbus_are_subchannels_present() was removed in commit
c967590457ca ("scsi: storvsc: Fix a race in sub-channel creation that can cause panic").

Remove this dead code, and the utility function invoke_sc_cb() that it is
the only caller of.

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/1635191674-34407-1-git-send-email-mikelley@microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>