More graceful `@parent` directive handling [issue #4942]

[4.2] Simplifies `Factory::extendSection` method
[4.2] Removes `@parent` from content before returning from `Factory::yieldContent`
Merge pull request #4943 from jimrubenstein/patch-1

[4.2] Graceful `@parent` directive handling [#4942]
Shorten code.
Removed unused use statement