[core] move locator Get<Type> definitions to instance.hpp (#10711)

This commit moves the locator `GetProvider::Get<Type>()` and its
related `Timer` and `Tasklet` methods to `instance.hpp`. With this
change we can remove `locator_getters.hpp` header fully thus
simplifying the code.

src/core/thread/dua_manager.cpp

@@ -37,7 +37,6 @@
 
 #include "common/as_core_type.hpp"
 #include "common/code_utils.hpp"
-#include "common/locator_getters.hpp"
 #include "common/log.hpp"
 #include "common/settings.hpp"
 #include "instance/instance.hpp"