Revert "[LE unicast] The isBusy state should involve the state of sub device"

This reverts commit 4b2b34ee.

Reason for revert: According to b/253592180#comment6, we should hide the concept of member sets of a CSIP pairing or joining a group. The UI did not gray out device preference when the member is busy.

Change-Id: Iae13955f34222854f7b1f6b767e9aed09cb3ba32