Same field in both goto_statet and goto_symex_statet

[JohnDumbell]

In a recent refactor 'source' got moved to a parent class and as I couldn't see any immediate reason for it to shadow the field I guess it was left here accidentally.

If there is a reason, just say and I'll close the PR.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: d22ce1e).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/100482734