Remove unused BluetoothLowEnergyPeripheralManagerBridge::adapter_

This pointer has been found to dangle, but it's not even used so we can
simply remove it.

Bug: 330009945
Change-Id: Icf51202adf4b7891aa52529534062d0373703044
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5404917


Auto-Submit: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Nina Satragno <nsatragno@chromium.org>
Reviewed-by: Nina Satragno <nsatragno@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279909}