[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/api/thread_api.cpp

@@ -39,8 +39,8 @@
 
 #include "common/as_core_type.hpp"
 #include "common/debug.hpp"
-#include "common/locator_getters.hpp"
 #include "common/uptime.hpp"
+#include "instance/instance.hpp"
 #include "thread/version.hpp"
 
 using namespace ot;