Parser: remove obsolete handling of else inside lib struct

[HertzDevil]

A long time ago, ifdef and else used to be special keywords inside lib structs and unions:

lib LibC
  struct Foo
    ifdef some_flag
      a : Int32
    else
      a : Float64
    end
  end
end

Today the above would be written as:

lib LibC
  struct Foo
    {% if flag?(:some_flag) %}
      a : Int32
    {% else %}
      a : Float64
    {% end %}
  end
end

So the handling for the else keyword in this context can be dropped. This doesn't affect the parser, but it does expose a syntax error in the top-level phase (detected only after macro interpolation) to the formatter.