audio/tuxguitar: Several changes

Port changes:

• Remove unnecesary -Dos.name={os-name} passed to Java VM
• Remove Gecko
• Remove the browser-ftp plugin
• Share the soundfont with FluidSynth, update pkg-message accordingly

PR: 196036
Submitted by: Reuben Kebreau <kyamashita@openmailbox.org> (-Dos.name= removal)
Submitted by: w.schwarzenfeld@utanet.at (Gecko/browser-ftp removal)