Remove unused quantifier_exprt default constructor

[tautschnig]

It made use of the deprecated symbol_exprt() constructor and was never used
anyway.

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[smowton]

If we don't get a test-gen compile failure

[allredj]

Passed Diffblue compatibility checks (cbmc commit: 7ff7c27).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/85998882