💥🔥 Remove deprecated #client_thread attr_reader

I don't know what the original purpose for `#client_thread` was, and it
was deprecated by v0.4.0.  There's nothing particularly special about
the Thread that created the client instance.  Net::IMAP doesn't use it,
clients can use multiple threads, and users of Net::IMAP can track this
for themselves if they need to.

lib/net/imap.rb

@@ -944,21 +944,13 @@ def initialize(host, port: nil, ssl:  nil,
       @sock = tcp_socket(@host, @port)
       start_tls_session if ssl_ctx
       start_imap_connection
-
-      # DEPRECATED: to remove in next version
-      @client_thread = Thread.current
     end
 
     # Returns true after the TLS negotiation has completed and the remote
     # hostname has been verified.  Returns false when TLS has been established
     # but peer verification was disabled.
     def tls_verified?; @tls_verified end
 
-    def client_thread # :nodoc:
-      warn "Net::IMAP#client_thread is deprecated and will be removed soon."
-      @client_thread
-    end
-
     # Disconnects from the server.
     #
     # Related: #logout, #logout!