Revert "Class instance should be also colorable on IRB"

This reverts commit 6669c966d2.

It seems to make tests fail... let me fix this later.

lib/irb/color.rb

@@ -51,7 +51,7 @@ module IRB # :nodoc:
           obj.all? { |k, v| inspect_colorable?(k) && inspect_colorable?(v) }
         when Array
           obj.all? { |o| inspect_colorable?(o) }
-        when String, Symbol, Regexp, Integer, Float, FalseClass, TrueClass, NilClass, Class
+        when String, Symbol, Regexp, Integer, Float, FalseClass, TrueClass, NilClass
           true
         else
           false

test/irb/test_color.rb

@@ -53,7 +53,6 @@ module TestIRB
         ['foo', :bar] => true,
         { a: 4 } => true,
         /reg/ => true,
-        Struct => true,
         Object.new => false,
         Struct.new(:a) => false,
         Struct.new(:a).new(1) => false,