Removed setDisplayNumber as it was not used.

SetDisplayNumber was called with mPhoneNumber parameter
that was not set at that time so null and the function
would only return -1 so it does not do anything.

Tag: #feature
Bug: 190441152
Test: atest BluetoothInstrumentationTests
Change-Id: Ie63e54bf99ef8e18b9acaebba0ef9aca1763d699