ConcurrentModificationException on platform startup

Reverted "foreach" that introduced regression.

Change-Id: Idc93603846b97af03bbee0607461f90ad74242fd
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/186610
Tested-by: Platform Bot <platform-bot@eclipse.org>