Remove symbol generator from add_axioms_for_* that do not use it

[tautschnig]

The fresh_symbol parameter was never used in these functions and is thus
removed.

@romainbrenguier Existing PRs of yours might make this one redundant, happy to close it and wait for them to be merged.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 91759a4).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/90992690