Shortcodes: use has_shortcode to check if shortcode exists

has_shortcode was introduced in 3.6. It's now safe to use in Jetpack
Contact Form: remove old conditional, oops

Error introduced in  9cc9561fdf596e7e2856f2e0c568c12b1dd1a5e5
Merge pull request #239 from jeherve/has_shortcode

Shortcodes: use has_shortcode to check for shortcodes in content