Activator was only used to access and track an OSGi immediate component
defined in the same bundle. Replace this by a simple static method.

Change-Id: I3c4e0514e07c9090d81dfe5eefe1b5320e3b66c8
Signed-off-by: Lars Vogel <Lars.Vogel@vogella.com>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/176261
Tested-by: Platform Bot <platform-bot@eclipse.org>