Revert part of "Start GitHub Actions" (4ce3997c)

Because some changes have been overridden.

lib/mruby/build.rb

@@ -276,14 +276,6 @@ EOS
       end
     end
 
-    def cygwin_filename(name)
-      if name.is_a?(Array)
-        name.flatten.map { |n| cygwin_filename(n) }
-      else
-        `cygpath -w "#{filename(name)}"`.strip
-      end
-    end
-
     def exefile(name)
       if name.is_a?(Array)
         name.flatten.map { |n| exefile(n) }

travis_config.rb

-MRuby::Build.new('debug') do |conf|
-  toolchain :gcc
-  yacc.command = ENV['YACC'] || 'bison'
-  enable_debug
-
-  # include all core GEMs
-  conf.gembox 'full-core'
-  conf.cc.flags += %w(-Werror=declaration-after-statement)
-  conf.compilers.each do |c|
-    c.defines += %w(MRB_GC_STRESS MRB_GC_FIXED_ARENA MRB_METHOD_CACHE)
-  end
-
-  build_mrbc_exec
-end
-
 MRuby::Build.new('full-debug') do |conf|
   toolchain :gcc
-  yacc.command = ENV['YACC'] || 'bison'
   enable_debug
 
   # include all core GEMs
@@ -28,7 +12,6 @@ end
 
 MRuby::Build.new do |conf|
   toolchain :gcc
-  yacc.command = ENV['YACC'] || 'bison'
 
   # include all core GEMs
   conf.gembox 'full-core'
@@ -42,7 +25,6 @@ end
 
 MRuby::Build.new('cxx_abi') do |conf|
   toolchain :gcc
-  yacc.command = ENV['YACC'] || 'bison'
 
   conf.gembox 'full-core'
   conf.cc.flags += %w(-fpermissive)