sslutil: stop checking for web.cacerts=! (BC)

The previous patch stopped setting web.cacerts=! to indicate
--insecure.

That left user configs as the only source that could introduce
web.cacerts=!.

The practical impact of this patch is we no longer honor
web.cacerts=! in configs. Instead, we always treat web.cacerts
as a path. The patch is therefore technically BC. However,
since I don't believe web.cacerts=! is documented, it should be
safe to remove. a939f08fae9c (which introduced --insecure) has
no indication that web.cacerts=! is anything but an implementation
detail, reinforcing my belief it can be removed without major
debate.

mercurial/sslutil.py

@@ -254,9 +254,6 b' def sslkwargs(ui, host):'
         return kws
 
     cacerts = ui.config('web', 'cacerts')
-    # TODO remove check when we stop setting this config.
-    if cacerts == '!':
-        return kws
 
     # If a value is set in the config, validate against a path and load
     # and require those certs.