[IMP] assets: remove css files split

Before this commit, our asset bundles had a special behaviour for
bundles of type css: because of an old limitation in IE9, css files had
to be split to make sure they had less than 4095 rules per stylesheets.

Since we no longer support IE9, this code can be removed, which should
slightly simplify the bundle code, and should slightly reduce the number
of requests required to properly load odoo.

closes odoo/odoo#27933

doc/reference/javascript_reference.rst

@@ -129,9 +129,8 @@ Here is what happens when a template is rendered by the server with these direct
       be replaced by a list of script tags pointing to the js files
 
 - if we are not in *debug=assets* mode,
-    - the css files will be concatenated and minified, then splits into files
-      with no more than 4096 rules (to get around an old limitation of IE9). Then,
-      we generate as many stylesheet tags as necessary
+    - the css files will be concatenated and minified, then a stylesheet tag is
+      generated
     - the js files are concatenated and minified, then a script tag is generated
 
 Note that the assets files are cached, so in theory, a browser should only load