Remove unused string for Bluetooth Active device summary

The string with ID "bluetooth_audio_active_device_summaries" is not
needed anymore. It has been replaced / superseded by strings with IDs
"bluetooth_active_battery_level" and "bluetooth_active_no_battery_level".

Bug: 74134939
Bug: 73007419
Test: make -j40 RunSettingsRoboTests
      make -j40 RunSettingsLibRoboTests

Change-Id: I929221f93806a50e3d61d50ea194d06eeecdbd24