overlay: drop overlay-helper

[spike-rabbit]

The current overlay-helper has some flaws:

• it adds a wrapper and an API without providing additional value or less code
• it mixes different use cases which leads to hard to read code
• it modifies constant values which could lead theoretically to weird runtime behavior
• the predefined position names are wrong

In most cases, the overlay-helper function can just be replaced with direct cdk calls.
For tooltips and popovers a a new contextual-overlay is potentially the best approach,
as those components have basically the same requirements on terms of overlay location.