Fix backwards compatibility with enabled plugins from the KF6 build

In KF6 the ID field is now useless, and the actual ID is taken from the plugin target name. In order to keep backwards compatibility (plugin ID is used to store the currently enabled plugins) we must rename the plugin target names to match the old plugin IDs.

Since the ID metadata field now does nothing, we can remove it.

Test-plan:

• Have a KF5 build & KF6 build of KTorrent
• Delete the [Plugins] section of ~/.config/ktorrentrc
• Open the KF5 version of KTorrent and enable some plugins
• Open the KF6 version of KTorrent

Previously:

• Only the default info-widget plugin was enabled the in KF6 version

Now:

• The same plugins are enabled for KF5 and KF6