Revert "Reland "[Clang][SemaCXX] Add unused warning for variables declared in...

Revert "Reland "[Clang][SemaCXX] Add unused warning for variables declared in condition expressions""

This reverts commit 92023b15.

Further issues were uncovered:
- False positive -Wunused-variable on function-static variables with the
  used attribute. https://github.com/ClangBuiltLinux/linux/issues/1926
  A test case needs to be added for this.
- Request for a new flag for these.
  https://reviews.llvm.org/D152495#4628877
  https://reviews.llvm.org/D152495#4628887