Apply some IWYU suggestions

This is not nearly all of them, because IWYU does not support the
way Catch2 manages includes -- it expects that non-system includes
are done using `#include "foo/bar/baz.hpp"`, while Catch2 uses
`<foo/bar/baz.hpp>`. This causes trouble, because IWYU suggests
removing every single internal header, and then adding them again,
but using `""` in the include directive... the resulting suggestions
cannot be used without a lot of manual work, as they are largely
bogus.

For bonus points, IWYU also _loves_ to suggest kinda-random stdlib
headers for `size_t` and similar. Still, the resulting inclusion
graph is somewhat better than it was before.

src/catch2/internal/catch_assertion_handler.cpp

@@ -7,8 +7,6 @@
  */
 
 #include <catch2/internal/catch_assertion_handler.hpp>
-#include <catch2/catch_assertion_result.hpp>
-#include <catch2/interfaces/catch_interfaces_runner.hpp>
 #include <catch2/interfaces/catch_interfaces_config.hpp>
 #include <catch2/internal/catch_context.hpp>
 #include <catch2/internal/catch_enforce.hpp>