Remove unused capture

Because `Regexp#to_s` add `(?-mix:)`, it is not unnecessary.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62250 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/transform_mjit_header.rb

@@ -9,7 +9,7 @@ PROGRAM = File.basename($0, ".*")
 
 module MJITHeader
   ATTR_VALUE_REGEXP  = /[^()]|\([^()]*\)/
-  ATTR_REGEXP        = /__attribute__\s*\(\((#{ATTR_VALUE_REGEXP})*\)\)/
+  ATTR_REGEXP        = /__attribute__\s*\(\(#{ATTR_VALUE_REGEXP}*\)\)/
   FUNC_HEADER_REGEXP = /\A(\s*#{ATTR_REGEXP})*[^\[{(]*\((#{ATTR_REGEXP}|[^()])*\)(\s*#{ATTR_REGEXP})*\s*/
   TARGET_NAME_REGEXP = /\A(rb|ruby|vm|insn|attr)_/