chore: Remove build.py

[albireox]

Installing the project now fails with error

Installing the current project: sdss-valis (0.1.1-alpha)
Preparing build environment with build-system requirements poetry>=1.0.0
/tmp/tmpfmj9ebia/.venv/bin/python: can't open file '/home/runner/work/valis/valis/build.py': [Errno 2] No such file or directory

Command ['/tmp/tmpfmj9ebia/.venv/bin/python', '/home/runner/work/valis/valis/build.py'] errored with the following return code 2
Error: Process completed with exit code 1.

even if the build.py file exists. This may be related to the new python-build package that is part of the setuptools echosystem, or a change in Poetry. Since build.py is just a placeholder from the python-template and we don't use it in Valis (we don't have any C extensions), the easiest is to remove it.