Remove styling for .form-control:focus:invalid

With these rules, when users click on an input with specific type (e.g. url, email,...), even if they haven't start typing, the input will be turned red. This is because the input is invalid (in this case, empty) at the focus moment. Demonstration: http://jsfiddle.net/9tndpu8o

This is not a good UX practice. Users properly want their inputs validated only after an event like blur or submit.

I suggest that we remove these two rules. If a developer need to highlight an invalidated input, s/he can add `has-error` class to the input's `.form-group` container on a more appropriate event like blur or submit.