Remove contrib/build_sysimg.jl (#30225)
* Remove contrib/build_sysimg.jl
Users should instead use [PackageCompiler.jl](https://github.com/JuliaLang/PackageCompiler.jl) as it is more fully-featured and better maintained.
* Modify `devdocs/sysimg.md` to just forward users to `PackageCompiler`.
* Remove `build_sysimg()` invocation from `doc/make.jl`
