Remove unnecessary assignment in String#upto

Co-authored-by: taiyoslime <t@iyosli.me>
Co-authored-by: smallkirby <ssmallkirby@gmail.com>

mrbgems/mruby-string-ext/mrblib/string.rb

@@ -422,7 +422,6 @@ class String
     # both edges are all digits
     bi = self.to_i(10)
     ei = max.to_i(10)
-    len = self.length
     if (bi > 0 or bi == "0"*len) and (ei > 0 or ei == "0"*maxlen)
       while bi <= ei
         break if exclusive and bi == ei