[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/common/notifier.cpp

@@ -37,8 +37,8 @@
 #include "common/array.hpp"
 #include "common/code_utils.hpp"
 #include "common/debug.hpp"
-#include "common/locator_getters.hpp"
 #include "common/log.hpp"
+#include "instance/instance.hpp"
 
 namespace ot {