Revert "add assertion for misuse of AbstractContextBuilder::openContext"

This usage is apparently sometimes used in the CPP lang plugin,
I have to investigate whether it's a bug there or whether
it depends on this usage implicitly, maybe as requirement for
contexts in macro expansions...

To unbreak master, lets revert this for now.

This reverts commit b494fac2293237ab095f040cace27b7f3e899d80.

BUG: 275136