Do not duplicate what c_type_as_string takes care of

[tautschnig]

With the exception of "bool", the type string output of the C front-end can be
used in the C++ front-end; the C front-end in turn uses c_type_as_string.

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[allredj]

Passed Diffblue compatibility checks (cbmc commit: f030b60).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/87954055

[martin-cs]

I think this works. I know there are some differences between the two in terms of overloading vs. coercion but that should have all been resolved before we've got to this bit of code.