Revert "Create presym files for each build target" (58ba883e)

Due to the above changes, it may not work with the existing build
configurations in cross-compilation (even if we can build without presym),
therefore revert it once (ref https://github.com/mruby/mruby/pull/5202#issuecomment-735412643). Sorry for the lack of consideration.

lib/mruby/build.rb

@@ -373,14 +373,6 @@ EOS
     def libraries
       [libmruby_static]
     end
-
-    def presym_file
-      "#{build_dir}/presym"
-    end
-
-    def presym_inc
-      "#{build_dir}/include/mruby/presym.inc"
-    end
   end # Build
 
   class CrossBuild < Build