cinnamon-session-check-accelerated and its helper are still unused

I asked this in #151 back in 2022; it got no reply and the stale bot closed it in April
2026, so here it is again with current findings.

tools/meson.build on master still builds and installs both
cinnamon-session-check-accelerated and cinnamon-session-check-accelerated-helper into
libexecdir, but git grep over master of cinnamon-session and of Cinnamon finds no
caller. The mechanism that would use them — IsRunnableHelper= / FallbackSession= in
csm-session-fill.c — is implemented, but no shipped .session file sets either key.

They are also redundant by now: Cinnamon detects this itself at runtime in js/ui/main.js
via backend.is_rendering_hardware_accelerated(), which in Muffin
(meta_renderer_is_hardware_accelerated()) already returns FALSE for LLVMPIPE, SOFTPIPE
and SWRAST.

Can they be dropped from the build? What to do when there is no usable GL at all is a
separate question, which I filed as linuxmint/Cinnamon#13996

Start in tools/meson.build and inspect the entries for cinnamon-session-check-accelerated and its helper. Confirm the absence of callers by checking csm-session-fill.c and the shipped session files, then review the existing runtime check in js/ui/main.js. Done means the unused programs are no longer built or installed, with the project build still succeeding.