Remove unnecessary MethodAttr#param_seq as it's always nil and not used

- `MethodAttr` is a superclass of `AnyMethod` and `Attr`
- `AnyMethod` defineds its own `param_seq` method so this is redundant
  for it
- The only place that calls `param_seq` is in `Darkfish`'s class page,
  which only calls it on `AnyMethod` as `Attr` is rendered in a different
  section
  - It doesn't really make sense for attributes to have a param_seq
    anyway

Author: st0012

lib/rdoc/code_object/method_attr.rb

@@ -63,12 +63,6 @@ class RDoc::MethodAttr < RDoc::CodeObject
 
   attr_reader :arglists
 
-  ##
-  # Pretty parameter list for this method
-
-  attr_reader :param_seq
-
-
   ##
   # Creates a new MethodAttr from token stream +text+ and method or attribute
   # name +name+.
@@ -91,7 +85,6 @@ def initialize text, name
     @arglists     = nil
     @block_params = nil
     @call_seq     = nil
-    @param_seq    = nil
     @params       = nil
   end