Bug#750205: Do not override debian compilation flags (dpkg)

[Mathieu Malaterre]

Package: libvtk6
Severity: important
Version: 6.1.0+dfsg-3
Tags: patch

Please remove buildsystem (cmake) specific build flags, it makes it
hard to control optimization flags.

  -DCMAKE_BUILD_TYPE=RelWithDebInfo \

should read:

  -DCMAKE_BUILD_TYPE=None \