Clear up the obsoleted cascading signal strength polling logic

The cascading polling logic were used on devices before
unsolicited signal reporting are supported. It has been
obsoleted now since all devices should support signal
reporting with radio hal after v1.2.

Bug: 178429976
Test: atest SignalStrengthControllerTest
Change-Id: Id82800f2bf9f025978d3e4271fb010e31aad242a