keg: also prune opt.

If there's dead symlinks in there they should be removed.

Library/Homebrew/keg.rb

@@ -66,7 +66,7 @@ class Keg
   INFOFILE_RX = %r{info/([^.].*?\.info|dir)$}
   TOP_LEVEL_DIRECTORIES = %w[bin etc include lib sbin share var Frameworks].freeze
   ALL_TOP_LEVEL_DIRECTORIES = (TOP_LEVEL_DIRECTORIES + %w[lib/pkgconfig share/locale share/man opt]).freeze
-  PRUNEABLE_DIRECTORIES = %w[bin etc include lib sbin share Frameworks LinkedKegs var/homebrew/linked].map do |dir|
+  PRUNEABLE_DIRECTORIES = %w[bin etc include lib sbin share opt Frameworks LinkedKegs var/homebrew/linked].map do |dir|
     case dir
     when "LinkedKegs"
       HOMEBREW_LIBRARY/dir