feat(jqLite): remove the attribute for .attr(attribute, null)

This change aligns jqLite with jQuery.

Also, the extra `2` second parameter to `setAttribute` has been removed;
it was only needed for IE<9 and latest jQuery doesn't pass it either.

Ref #15126

BREAKING CHANGE: Invoking `elem.attr(attributeName, null)` would set the
`attributeName` atribute value to a string `"null"`, now it removes the
attribute instead.

To migrate the code follow the example below:

Before:

elem.attr(attributeName, null);

After:

elem.attr(attributeName, "null");