python: remove package_available? logic

It was added in https://github.com/Homebrew/legacy-homebrew/pull/37345 for
a use case discussed in https://github.com/Homebrew/legacy-homebrew/pull/37340
that never seems to have actually emerged. I can find no reference of it
ever having been used, despite it existing for 2+ years here.

Library/Homebrew/language/python.rb

@@ -71,10 +71,6 @@ module Language
       ]
     end
 
-    def self.package_available?(python, module_name)
-      quiet_system python, "-c", "import #{module_name}"
-    end
-
     # Mixin module for {Formula} adding virtualenv support features.
     module Virtualenv
       def self.included(base)