Removed unsafe eval() from compressed treeselectmenu script

referring to issue #3837:

> I'm using CSP on my server. Sadly, I have to allow unsafe-eval because of Joomla. There are more secure alternatives to eval():
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval#Don.27t_use_eval.21
See also http://www.w3.org/TR/CSP/#script-src
do not see a reason for such changes, `secure || JSON.secure` do same as your code :wink: