Remove duplicate class

The com.android.internal.telephony.ExponentialBackoff class is defined
both in framework/base and framework/opt/telephony. The one in base
appears first in boot class path and therefore is the one that gets
resolved by the VM. Remove the one in telephony.

Bug: 111116543
Test: m
Change-Id: I4855fa5b18203b08ff36071b5e562918d91120c6