prerm is in tightvncserver 1.3.10-0ubuntu3.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 | #!/bin/sh
set -e
if [ "$1" = "remove" ] ; then
    BIN=/usr/bin
    update-alternatives --remove \
	vncserver $BIN/tightvncserver
    update-alternatives --remove \
	Xvnc $BIN/Xtightvnc
    update-alternatives --remove \
        vncpasswd $BIN/tightvncpasswd
    update-alternatives --remove \
        vncpasswd $BIN/tightvncpasswd
fi
exit 0
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