[Users] Remove default variable guess from Multipole

Hi,

When the Multipole thorn was first written at Penn State for debugging, we were using it with WeylScal4 and to avoid incompatibilities with early parameter files, when the "variables" parameter was added we decided that if it was left blank, it would default to the WeylScal4 Psi4 variable. This is not a good logical default, as the thorn provides generic mode decomposition. I would like to remove this, and the patch is attached. OK to commit?