fluid-synth: Remove glib and static library

[FtZPetruska]

Starting version 2.6.0, fluid-synth dropped glib support, instead relying on C++11 features.

Also remove a handful of unneeded flags:

• enable-libinstpatch was removed along glib
• enable-dart is only considered when targeting OS/2
• enable-lash had already been removed previously
• enable-sdl2 is updated to sdl3

Finally, stop building a static library, so dependencies do not link to it and miss updates when no revision bump is needed.

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

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