[TASK] Remove dead code in ExtensionConfiguration class

This patch:

* Removes dead code which deals with setting an extension
   configuration option by path (which is not supported!)
* Removes an unused parameter to pass the path.
* Adjusts tests and usages of the class to no longer pass
  an empty string as $path argument.

Releases: master
Resolves: #89957
Change-Id: Ia74a9d9994056599006e375ec63c32dcd8e4c403
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62650


Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>