Remove `brew cask update` from manpage.

Library/Homebrew/manpages/brew-cask.1.md

@@ -104,9 +104,6 @@ names, and other aspects of this manual are still subject to change.
     Uninstall the given Cask. With `--force`, uninstall even if the Cask
     does not appear to be present.
 
-  * `update`:
-    For convenience. `brew cask update` is a synonym for `brew update`.
-
   * `zap` <token> [ <token> ... ]:
     Unconditionally remove _all_ files associated with the given Cask.
 

manpages/brew-cask.1

@@ -89,9 +89,6 @@ If \fItoken\fR is given, summarize the staged files associated with the given Ca
 \fBuninstall\fR or \fBrm\fR or \fBremove\fR [\-\-force] \fItoken\fR [ \fItoken\fR \.\.\. ]: Uninstall the given Cask\. With \fB\-\-force\fR, uninstall even if the Cask does not appear to be present\.
 .
 .IP "\(bu" 4
-\fBupdate\fR: For convenience\. \fBbrew cask update\fR is a synonym for \fBbrew update\fR\.
-.
-.IP "\(bu" 4
 \fBzap\fR \fItoken\fR [ \fItoken\fR \.\.\. ]: Unconditionally remove \fIall\fR files associated with the given Cask\.
 .
 .IP