Remove media.variables.inc

The media.variable.inc file provides wrapper functions around core's variable_* functions which make them easier to use. However, Media: YouTube only uses these functions once (to get defaults for the YouTube formatter) and I don't see any benefit to allowing people to change the variables for the defaults, since they can just enter whatever they want when setting up the formatters or change the default formatter configuration using media_youtube_file_default_displays_alter().