implot: Don't compile implot_demo.cpp

[OatmealDome]

implot_demo.cpp depends on functions from imgui_demo.cpp, which we do not have. Appears to cause linking errors only on my fork, but I figured I would upstream this anyway.

[Pokechu22]

It might be better for us to include imgui_demo.cpp, as it is useful as an example (and shouldn't have much costs otherwise). But removing this also seems like a valid approach.

[shuffle2]

Please remove it from compilation (ie this PR as-is) if it's never used/not needed. I think it takes a long time to compile (have just noticed console output stalling there, but a lot of stuff is compiling in parallel and I haven't profiled it, so not sure).