Fix for build size setting in .pronsolerc

The bed_size_x/y variables in .pronsolerc where unused and
caused a warning at startup. This sets build_dimensions
instead to the default value as this is the variable you
need to adapt when your printer has a bigger build area.

The bed_size variables were really misleading for me: I adapted them to the bed size of the Mendel, but nothing happened. I used the pronterface with the wrong bedsize for quite a while before I found out what setting I really had to change in my ~/.pronsolerc. I now use

    set build_dimensions 200x200x94

and it works like a charm :-)