uninstall: don't need kegs to exist for --force.

Otherwise there's an error which is a regression on previous
functionality. This mirrors `rm -f` which doesn't fail if a file doesn't
exist.

Library/Homebrew/cmd/uninstall.rb

@@ -21,6 +21,7 @@ module Homebrew
     kegs_by_rack = if ARGV.force?
       Hash[ARGV.named.map do |name|
         rack = Formulary.to_rack(name)
+        next unless rack.directory?
         [rack, rack.subdirs.map { |d| Keg.new(d) }]
       end]
     else