imagemagick: remove broken openmp option

This doesn't work for several reasons:

 - the option is not declared anywhere
 - the code in the class body checks for 'with-openmp', while the code
   in the install method checks for 'enable-openmp'
 - even after adding a 'with-openmp' option, trying to use it breaks the
   formula because it tries to access ENV.compiler in the class body.

Given that this has been the case for quite some time and nobody has
complained, I'm removing this code altogether.