Clean up #includes

Remove unused headers, and avoid relying on transitive inclusions

`include-what-you-use` has been very useful for this!

include/link/main.hpp

@@ -9,7 +9,6 @@
 #include <variant>
 #include <vector>
 
-#include "helpers.hpp"
 #include "linkdefs.hpp"
 
 // Variables related to CLI options

include/link/output.hpp

@@ -3,8 +3,6 @@
 #ifndef RGBDS_LINK_OUTPUT_H
 #define RGBDS_LINK_OUTPUT_H
 
-#include <stdint.h>
-
 #include "link/section.hpp"
 
 /*

include/link/section.hpp

@@ -14,7 +14,6 @@
 #include "linkdefs.hpp"
 
 #include "link/main.hpp"
-#include "link/patch.hpp"
 
 struct FileStackNode;
 struct Section;