Remove options that do not take effect when using custom-set-variable from the customize interface

[nawetimebomb]

The options listed below were removed from the customize interface since they don't take effect when using custom-set-variables as described on issue #325.

zenburn-use-variable-pitch
zenburn-scale-org-headlines
zenburn-scale-outline-headlines

Also changed the docstring and the README.md to reflect this.

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Thanks!

[bbatsov]

Thanks!