sdl: drop the transitional SDL2 build once libossia is on SDL3

[edumeneses]

Tracking issue so the transitional SDL2 build doesn't become permanent.

#27 ships SDL3 3.4.14 into $INSTALL_PREFIX/SDL3 while keeping SDL2 2.32.10 in $INSTALL_PREFIX/SDL2, because libossia still speaks the SDL2 API and score's score_assert_feature(sdl) turns a missing ossia::sdl2 into a hard failure on deployment builds. Once libossia is ported, SDL2 has no consumer left and should come out.

Blocked on

The libossia port (part of #6): the joystick/game-controller protocols, the SDL audio engine, cmake/deps/sdl.cmake's find_package(SDL2) → ossia::sdl2, and the $OSSIA_SDK/SDL2 prefix paths in OssiaOptions.cmake and score's CMakeLists.txt. There is also an open decision on the WASM leg — emscripten's sdl3 port exists at our EMSDK_VERSION pin but is marked experimental (details in #27 (comment)).

Then, in this repo

Mechanical once nothing links ossia::sdl2:

• delete Linux/sdl.sh, macOS/sdl.sh, MSYS/sdl.sh, AArch64/sdl.sh
• delete common/clone-sdl.sh
• drop export SDL_VERSION=2.32.10 from common/versions.sh, along with the comment block above it explaining why SDL2 is still there
• drop the ./sdl.sh lines from Linux/build-all.sh, macOS/all.sh, MSYS/all.sh, AArch64/build-all.sh
• README.md: SDL2 and SDL3 → SDL3
• consider renaming sdl3.sh → sdl.sh and SDL3_VERSION → SDL_VERSION at that point, so the scripts stop carrying a version number in their name

MSVC/sdl.ps1 is separately dead (pins Visual Studio 2017 and SDL 2.0.9, not wired into any workflow) — worth deleting whenever, independent of this.

Checks before merging the removal

• no find_package(SDL2) / SDL2:: / ossia::sdl2 left in libossia, score, or avendish (avendish/cmake/avendish.ossia.cmake and avendish.standalone.cmake also reference SDL2)
• score deployment build still passes score_assert_feature(sdl)
• the WASM leg still builds, whichever way the emscripten port decision goes

[jcelerier]

if we manager to show that a SDL3 build I'd rather directly modify the current sdl.sh so that they change to SDL3, I'm not sure it's necessary to have both. we can just make a build of libossia & score pinning to a temp tag here to validate