Do not handle float and double to string operations in the string solver

[forejtv]

A follow-up from #4633, this PR is a shift towards handling more of the string operations in Java. All it does is to remove a couple of bindings from Java methods to string solver, so that the actual Java implementation is used.

There is an accompanying PR for java-models-library: diffblue/java-models-library#23.

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• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
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[thk123]

Less code is good if no tests start failing :)

[martin-cs]

Yeah... less axiom instantiation built in is generally a good thing.

[tautschnig]

@forejtv cprover_string_concat_double_func now needs to be removed from irep_ids.def as it is no longer used.

[forejtv]

This is now ready to go, I'll just wait for diffblue/java-models-library#23 to be approved so that we don't lose any existing functionality.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: af7bdaa).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/111782136

[tautschnig]

@forejtv I'll leave it to you to merge this so that you can make sure that the submodule pointer points to exactly the right commit.