Clean up old scripts/build tools

Remove the old sonnet script, as well as the obsolete
make-nix-release.sh and Makefile.

The last one is maybe more borderline, but it's ill-maintained and has
no consumers as best I can tell.

poetry/utils/env.py

@@ -1415,10 +1415,6 @@ def _bin(self, bin: str) -> str:
             # This is especially true when installing Python with
             # the official installer, where python.exe will be at
             # the root of the env path.
-            # This is an edge case and should not be encountered
-            # in normal uses but this happens in the sonnet script
-            # that creates a fake virtual environment pointing to
-            # a base Python install.
             if self._is_windows:
                 if not bin.endswith(".exe"):
                     bin_path = self._bin_dir / (bin + ".exe")