cubicweb #2065651 let's stop spitting xhtml [validation pending]

tier js libraries don't care anyway and most of the time we use force-html-content-type=yes

also it can lead to very hard-to-find bugs

priorityimportant
typeenhancement
appeared in<not specified>
done in3.17.0
load1.000
load left0.000
closed by#9251:7b26fe71404f
patch