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/020-TestCase-1.cpp

@@ -6,7 +6,7 @@
 // Let Catch provide main():
 #include <catch2/catch_default_main.hpp>
 
-#include <catch2/catch.hpp>
+#include <catch2/catch_test_macros.hpp>
 
 TEST_CASE( "1: All test cases reside in other .cpp files (empty)", "[multi-file:1]" ) {
 }