[js] remove 3.9 bw compat (where apparently unused) - cubicweb.ajax.js - loadxhtml: form.callback support removal - removal of top-level functions: preprocessAjaxLoad, reloadBox, replacePageChunk, loadxhtml - cubicweb.compat.js: - map is undeprecated (jquery.map being not an acceptable replacement) - removal of noop, contains, findValue, filter, addElementClass, removeElementClass, hasElementClass, KEYS mapping - htmlhelpers.js: use non-deprecated functions cw.urlEncode - cubicweb.js: - removal of startsWith and endsWith monkeypatches - note deprecated but still used stuff (for action) - test_utils.js: use cw.utils.sliceList instead of global function Closes #2782004.

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