[core] optimize #include directives in core cpp files (#10717)

This commit removes unnecessary `#include` directives in core `cpp`
files that already include `instance.hpp`. The `instance.hpp` header
itself includes all necessary OT core headers and definitions as they
are contained within an `ot::Instance`.

src/core/thread/tmf.cpp

@@ -34,7 +34,6 @@
 #include "thread/tmf.hpp"
 
 #include "instance/instance.hpp"
-#include "net/ip6_types.hpp"
 
 namespace ot {
 namespace Tmf {