Remove non-functioning 'DevicesFactoryHalInterface::getHalPids'

Since audioserver lacks SELinux permission to list services, the
code for retrieving HAL PIDs via DevicesFactoryHal does not
work. This has been addressed a long time ago by pushing the HAL
PIDs from the system server (see aosp/1183075). However, the
non-functioning code was still lingering around.

Bug: 298548691
Test: adb shell kill -9 <audioserver pid>
Change-Id: I3fec23e9ad9072ea46cb76c3f9a280123f1c8007