build: remove Ion.egg-info and build_sources.sh

Python eggs are no longer supported: they have been entirely replaced by
wheels. Therefore, carrying around an `Ion.egg-info` and having to build
sources is highly unnecessary.

Author: krishnans2006

.github/workflows/ci.yml

@@ -104,10 +104,6 @@
                     "name": "Regenerate workflow YAML file and check for changes",
                     "run": "./scripts/build_ensure_no_changes.sh ./ci/regen-workflow.py"
                 },
-                {
-                    "name": "Build sources",
-                    "run": "./scripts/build_ensure_no_changes.sh ./scripts/build_sources.sh"
-                },
                 {
                     "name": "Check for changes to CI spec",
                     "run": "./scripts/build_ensure_no_changes.sh ./ci/regen-workflow.py"