[clang] [analyzer] Remove impossible BugType from CStringChecker (PR #152163)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

The failure of the CI jobs is unrelated to this commit; it seems that the tests were broken on _main_ at the moment when the CI jobs tested this commit. As main was stabilized since that time, re-running the CI jobs should be successful.

https://github.com/llvm/llvm-project/pull/152163