AMA installer wrongly assumes if config file exists then ARC is installed even though it isn't

Agent assumes that if ArcSettingsFile exists then ARC is installed. Problem is that removing the ARC RPM doesn't remove ArcSettingsFile.

This caused a bit of confusion on some VMs where a while ago ARC was accidentally installed on Azure VMs but was later removed. AMA picked old proxy configurations from that file that prevented AMA from sending logs.

https://github.com/Azure/azure-linux-extensions/blob/master/AzureMonitorAgent/agent.py#L797-L800

Thanks.

Start in AzureMonitorAgent/agent.py at lines 797-800 and inspect how ArcSettingsFile is used to infer ARC installation. Reproduce the case where the configuration file remains after the ARC RPM is removed, then verify that AMA no longer loads stale proxy settings when ARC is absent.

Written by the indexing model from the issue text.