[web/views/formrenderer] do not use `cubicweb:target` attribute on form (closes #5534074) The `cubicweb:target` is a flag indicating that the form data (with file input) shall be posted inside an iframe - this is a well known "ajax-like" workaround to post files with browsers that do not support `FormData` (a.k.a IE<10). The `cubicweb:target` was introduced when CW used the "xhtml strict" doctype. Now that CW uses the "html5" doctype, this namespaced attribute is no longer necessary and the iframe can be generated directly. Before this patch, CW inserts the `cubicweb:target` attribute in the form DOM element (server side) and `setFormsTarget()` updates the DOM with a new `<iframe>` element (client side). Now, CW inserts the `<iframe>` DOM element directly (server side), making `setFormsTarget` useless.

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