graphics/pstoedit: switch to using the default-version of ImageMagick

editors/calligra would pull in graphics/ImageMagick6 making it
incompatible with other software relying on ImageMagick7.

The ports framework already supports depending on a default-version of
ImageMagick using bsd.default-versions.mk's ${IMAGEMAGICK_DEFAULT}, and
a patch to support building graphics/pstoedit was added by mi@ in 2017
in 0d908edaeb50afbbc115905bcc9c9a241c59c839 - however the dependency
line was never adopted to it.

Reported by: Graham Perrin <grahamperrin@gmail.com>