bugfix: must do .update() after remove_returns

[kroening]

remove_returns inserts new instructions, and thus, .update() must be called.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: d09eb77).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/100346835

[kroening]

To see a reason why this is needed, look at #4001. This fails without the fix.

[tautschnig]

If remove_returns requires a call to .update(), why is remove_returns not made to call .update()?