Do not unnecessarily set ID_base_name

[tautschnig]

The value was set, but never read.

• Each commit message has a non-empty body, explaining why the change was made.
• My contribution is formatted in line with CODING_STANDARD.md.
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• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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[smowton]

@thk123 any usage in TG?

[tautschnig]

@thk123 any usage in TG?

In case there is: class_type.get_tag() would yield the same value, and would thus be the change required if this were to be merged.

[allredj]

Passed Diffblue compatibility checks (cbmc commit: 93c0c65).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/85561530

[thk123]

Nope - no usages of ID_base_name in TG 👍