Remove can-ajax import inside data-url behavior

[DesignByOnyx]

The data-url behavior allows you to pass a custom ajax transport. However, if a consumer decides to use their own transport for all connections, the can-ajax dependency is doesn't get shaken out of the deps tree due to the hard import in can-ajax.

I'm not sure how to best solve this, as each connection will be required to pass the desired ajax transport and would likely be a breaking change.

[justinbmeyer]

Folks could make their own data-url if they really needed this. Might be worth keeping data-url working by default. can-ajax isn't that big.

[justinbmeyer]

An alternative would be to make a data-url without the hard coding (data-ajax), but data-url basically extends data-ajax into data-url.