gnu: Remove "open source" from descriptions.
All software in Guix is Free Software.
* gnu/packages/aidc.scm (libdmtx)[description]: Remove "open source" string.
* gnu/packages/firmware.scm (seabios)[description]: Likewise.
* gnu/packages/libupnp.scm (libupnp)[description]: Likewise.
* gnu/packages/maths.scm (cubicle)[description]: Likewise.
* gnu/packages/mingw.scm (mingw-w64)[description]: Likewise.
* gnu/packages/game-development.scm (openmw)[description]: Likewise.
[synopsis]: Remove "Free software".