[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/tmf.cpp

@@ -33,7 +33,7 @@
 
 #include "thread/tmf.hpp"
 
-#include "common/locator_getters.hpp"
+#include "instance/instance.hpp"
 #include "net/ip6_types.hpp"
 
 namespace ot {