remove org2blog/wp-sourcecode-langs

`org2blog/wp-sourcecode-langs` duplicates a list of supported languages which depends entirely on the version of the WordPress SyntaxHighlighter being used, so it is impossible to maintain this list to be correct for all users, and it is unrealistic to expect users to maintain it to be accurate.

Additionally, the only effect the variable has is to force the shortcode to be `text` in case an language is chosen which is not in this list. In that case, forcing it to text is almost certainly not what the user would want or expect.

Therefore it is better to adhere to the DRY rule and just eliminate this duplication.