Remove catch.hpp

This was an old "include all" header, that we no longer want to be
usable, to make the include differences in new versions explicit.

We will introduce new "include all" headers later, in the form of
`catch_all.hpp`, `catch_matchers_all.hpp` and so on...

examples/120-Bdd-ScenarioGivenWhenThen.cpp

@@ -2,7 +2,7 @@
 
 // main() provided in 000-CatchMain.cpp
 
-#include <catch2/catch.hpp>
+#include <catch2/catch_test_macros.hpp>
 
 SCENARIO( "vectors can be sized and resized", "[vector]" ) {