Remove unneeded and unused TermControl2 class name (#17269)
Related to microsoft/terminal#12358
Summary of the issue:
For microsoft/terminal#12358, we discussed having a second `TermControl2` class name to distinguish terminals that do and don't support UIA notifications, but this was never implemented and there are no plans to do so.
Description of how this pull request fixes the issue:
Remove unneeded constant
Testing strategy:
Alpha testing
Known issues with pull request:
None known
