prerm is in ncbi-tools-x11 6.1.20170106-2.
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 | #!/bin/sh -e
update-alternatives --remove Cn3D /usr/bin/Cn3D-3.0
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