Split out CaseSensitive enum out of catch_common.hpp

Only very few places actually use it, so there is no need to have
it be central to everywhere.

src/catch2/internal/catch_common.hpp

@@ -27,11 +27,6 @@ std::ostream& operator<<(std::ostream&, Catch_global_namespace_dummy);
 
 namespace Catch {
 
-    struct CaseSensitive { enum Choice {
-        Yes,
-        No
-    }; };
-
     struct SourceLineInfo {
 
         SourceLineInfo() = delete;