[clang] [clang] Remove dead incremental Parser code (PR #102450)

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

> Your reasoning sounds right to me. Can we make sure we are not breaking `clang -fincremental-extensions`, too?

As far as I can tell, `-fincremental-extensions` should set the language option `IncrementalExtensions` which in turn is the default for `Preprocessor::IncrementalProcessing`. The few tests we have with `-fincremental-extensions` pass with this change.

https://github.com/llvm/llvm-project/pull/102450