Revert "Demos: changed demo script src because combining scripts with PHP resulted in wrong src in the build."

This reverts commit aded72456967c347521ec972693dfe6e3f391848.
Build: Concatenate source files in docs/demos/_assets/js into docs/demos/_assets/jquery.mobile.demos.js and sed the reference to _assets/js/ in docs/demos/*{html,php} to _assets/js/jquery.mobile.demos.js