dreamcast_shelf.rb: remove unnecessary gem lines.

- `mruby-binding-core` should be handle through dependency
- `mruby-inline-struct` is only for tests

build_config/dreamcast_shelf.rb

@@ -74,7 +74,6 @@ MRuby::CrossBuild.new("dreamcast") do |conf|
 
   conf.gem :core => "mruby-array-ext"
   conf.gem :core => "mruby-binding"
-  conf.gem :core => "mruby-binding-core"
   conf.gem :core => "mruby-catch"
   conf.gem :core => "mruby-class-ext"
   conf.gem :core => "mruby-cmath"
@@ -90,7 +89,6 @@ MRuby::CrossBuild.new("dreamcast") do |conf|
   conf.gem :core => "mruby-exit"
   conf.gem :core => "mruby-fiber"
   conf.gem :core => "mruby-hash-ext"
-  conf.gem :core => "mruby-inline-struct"
 #  conf.gem :core => "mruby-io"
   conf.gem :core => "mruby-kernel-ext"
   conf.gem :core => "mruby-math"