Remove unused `MRuby::Build#enable_{bin,}test=`
The writers seem to be unnecessary because `MRuby::Build#enable_{bin,}test`
are used from the beginning.
@@ -55,7 +55,6 @@ module MRuby
include LoadGems
attr_accessor :name, :bins, :exts, :file_separator, :build_dir, :gem_clone_dir
attr_reader :libmruby_objs, :gems, :toolchains, :gem_dir_to_repo_url
- attr_writer :enable_bintest, :enable_test
alias libmruby libmruby_objs
