This reverts commit 3f00c8b2d69ac6f06a803c046a61a9a104ba5790.

The recursive auto-expansion of elements without siblings in the project
explorer led to unintended behavior for non-folder/non-package elements,
which are not expected to be expanded even if they do not have siblings
(such as a class within a Java file). Until a better, context-solution
is found, this reverts the enablement of the auto-expansion
functionality in the project explorer.

Contributes to https://github.com/eclipse-platform/eclipse.platform.ui/issues/1063

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/2002