chore: remove src folder from source

[vishwarajanand]

src folder doesn't seem to be used in the repo, so it should be removed.

BREAKING_CHANGE_REASON="BC check detects a missing /src folder hence complaining"

[bshaffer]

This is being referenced in our release script and composer.json, so we will need to make sure that those are updated:

• in composer.json
• in release scripts (addressed in chore: remove manifest.json, Version.php, and ServiceBuilder.php release-please#1949)

[vishwarajanand]

Thanks for the tip, @bshaffer .
I also added a BC justification to ignore the missing /src logs in the BC checks. Old logs here

Also, I am not sure how to fix the release scripts. Hoping your PR #1949 addresses it.