don't lie about the default of User

unlike the isync wrapper, mbsync does not have a default for the IMAP
user. the remote user seldomly matches the local one, so "forwarding" it
is more confusing than helpful.

CCMAIL: 744389@bugs.debian.org

src/mbsync.1

@@ -245,7 +245,7 @@ Specify the TCP port number of the IMAP server.  (Default: 143 for IMAP,
 ..
 .TP
 \fBUser\fR \fIusername\fR
-Specify the login name on the IMAP server.  (Default: current local user)
+Specify the login name on the IMAP server.
 ..
 .TP
 \fBPass\fR \fIpassword\fR