Remove SSA thread suffix in goto trace address-of expressions

[tautschnig]

SSA thread suffixes (e.g., "!0", "@1") are internal SSA identifiers that should be removed when displaying trace information to users. Similar SSA suffix removal logic already existed in
src/goto-programs/graphml_witness.cpp (the remove_l0_l1 function), but this logic was not being applied to expressions in the goto trace formatting code.

Fixes: #902

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