graphics/libdmtx, graphics/dmtx-utils update to 0.7.5 and 0.7.6 respectively

[kbowling]

Summary

Update to new GitHub upstream. Remove unnecessary ImageMagick dependency from libdmtx. Increase dmtx-utils dependency to ImageMagick7.

This fixes dependency mayhem that has existed for over a year on my KDE5 box if trying to update to ImageMagick7 as ports sets as default.

Test Plan

https://people.freebsd.org/~kbowling/logs/libdmtx-0.7.5.log
https://people.freebsd.org/~kbowling/logs/dmtx-utils-0.7.6.log

[kbowling]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=241803 has done part of this