gpg: Do not print a hint to use the deprecated --keyserver option.

* g10/keyserver.c (keyserver_search): Remove a specialized error
message.

Dirmngr comes with a default keyserver and the suggestion to use
gpg --keyserver
is not good because that option is deprecated. An error message
"No keyserver available" is sufficient.

• GnuPG-bug-id: T4512
• Signed-off-by: Werner Koch <wk@gnupg.org>