left_side_userlist: Remove feature from frontend.

Fixes #23517.

While this feature was added to Zulip very early, it has been troubled
for most of that time; it never looked great visually, had a lot of
implementation complexity around resize.js, and has a weird model (a
setting that changes the UI only in certain window sizes).

This option is not commonly used; while a significant portion of users
have it enabled, many of them just don't use window sizes where it
actually has an effect. So it's not clear that it will be missed if
removed; we got very few bug reports when it was completely broken for
a few days after we first integrated the new left sidebar private
messages design.

Even with it no longer being broken, it does not work very well with
the addition of the new PMs section in the left sidebar. (Having two
scrollbars in the sidebar looks quite awkward.) The new private
messages section in the left sidebar also addresses some of the use
cases for always keeping the Users list always visible, even in narrow
windows.

This option is only removed from frontend for now. To make this
decision easily reversible, the backend code of this feature
is still kept.

zerver/models.py

@@ -1485,10 +1485,13 @@ class UserBaseSettings(models.Model):
     created after the change.
     """
 
-    # UI settings
+    ### Generic UI settings
     enter_sends = models.BooleanField(default=False)
 
-    # display settings
+    ### Display settings. ###
+    # left_side_userlist was removed from the UI in Zulip 6.0; the
+    # database model is being temporarily preserved in case we want to
+    # restore a version of the setting, preserving who had it enabled.
     left_side_userlist = models.BooleanField(default=False)
     default_language = models.CharField(default="en", max_length=MAX_LANGUAGE_ID_LENGTH)
     # This setting controls which view is rendered first when Zulip loads.