sslutil: restore old behavior not requiring a hostname argument (issue5210)

This effectively backs out changeset 1fde84d42f9c.

The http library behind ui.http2=true isn't specifying the hostname.
It is the day before the expected 3.8 release and we don't want to ship
a regression.

I'll try to restore this requirement in the 3.9 release cycle as part
of planned improvements to Mercurial's SSL/TLS interactions.

mercurial/sslutil.py

@@ -120,9 +120,6 b' def wrapsocket(sock, keyfile, certfile, '
       server (and client) support SNI, this tells the server which certificate
       to use.
     """
-    if not serverhostname:
-        raise error.Abort('serverhostname argument required')
-
     # Despite its name, PROTOCOL_SSLv23 selects the highest protocol
     # that both ends support, including TLS protocols. On legacy stacks,
     # the highest it likely goes in TLS 1.0. On modern stacks, it can