Revert the default call to firefox

This reverts commit 0928a37.

No, no, you guys are right in the comments for rapid7#2148. The call to
system is inside the else, but the tabbing made my eyes cross.
Sorry about that. Someday soon, @tabassassin will save us all from these
kinds of screw ups in mental parsing.

Author: Tod Beardsley

lib/rex/compat.rb

@@ -145,9 +145,6 @@ def self.open_browser(url='http://metasploit.com/')
 				end
 			end
 		end
-
-		# If nothing else worked, default to firefox
-		system("firefox #{url} &")
 	end
 end