Drop start feature/attribute as its going away

[ipetkov]

Motivation

Fixes #767

Checklist

• added tests to verify new behavior
• added an example template or updated an existing one
• updated docs/API.md (or general documentation) with changes
• updated CHANGELOG.md

[kadiwa4]

Are you sure that isize is the right type for the argc param and the return value? Usually it would be c_int.

[ipetkov]

IIRC when I checked c_int was an alias for isize. Either way, this doesn't matter too much, the tests are really checking that a custom source (no matter its contents) shows up in the right place, but we aren't really checking that things compile correctly (that's ultimately up to whomever is customizing what the dummy source the want)