build.rst doc is now empty and should be removed

[drew-parsons]

PR #1360 consolidated build documentation and left build.rst empty. But the (empty) file is still in the repo,
https://github.com/xtensor-stack/xsimd/blob/d0c4a8f4262cb844dd2ca58c331ed53a411f107b/docs/source/build.rst

I guess it should be removed. It sort of causes problems with the sphinx build, for instance triggering a warning

checking consistency... /home/drew/projects/mathlibs/build/xsimd/docs/source/build.rst: WARNING: document isn't included in any toctree [toc.not_included]

Is there any reason why it needs to stay as an empty file?

[JohanMabille]

Is there any reason why it needs to stay as an empty file?

I think it is a leftover, I'll remove it in the next cleanup PR.