Remove cprover_string_intern handling in the solver

[romainbrenguier]

This has never been tested and should rather be handled by the models,
it is a Java specific feature.

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[tautschnig]

@romainbrenguier The test java_intern in strings-smoke-tests is now failing.

[romainbrenguier]

@tautschnig

@romainbrenguier The test java_intern in strings-smoke-tests is now failing.

So I was wrong, it is tested. I added a model to fix the problem, and added a complementary test.