[ruby/racc] Remove ErrorSymbolValue reference

I cannot find where this is defined.  I'm guessing neither this
nor the else branch is ever hit.  I only found out when testing
VM changes, which had a bug that exposed this.

https://github.com/ruby/racc/commit/8983d0c134

lib/racc/grammar.rb

@@ -993,10 +993,6 @@ module Racc
         @to_s = '$end'
         @serialized = 'false'
         @string = false
-      when ErrorSymbolValue
-        @to_s = 'error'
-        @serialized = 'Object.new'
-        @string = false
       else
         raise ArgumentError, "unknown symbol value: #{value.class}"
       end