Remove LTE Rsrp Boost from SignalStrength

The RSRP boost is no longer used in SignalStrength,
and there are no major users of this value to
justify keeping the value around even though there is
no direct alternative. Thus, removing it from
SignalStrength.

Bug: 122846268
Test: compilation (code removal)
Change-Id: I2d4409986a64aa13379e2894457d000ae43f92ca