TargetPlatformHelper and TargetPlatformService keep separate caches for
target definition, resulting in a leak of TargetDefinition objects.

The leak happens when TargetDefinition.resolve() caches data in
TargetPlatformHelper.

This also allows to skip the explicit removal of a target selected for
removal from the TargetPlatformHelper's cache in
TargetPlatformPreferencePage.handleRemove() because the removal will
done when the change is applied and TargetPlatformService.deleteTarget()
is then called.

Fixes https://github.com/eclipse-pde/eclipse.pde/issues/1246