debugUnifyBottomAndReturnBottom: remove duplicate call to TermLike.mapVariables

[emarzion]

…er called twice.

This code issue was discovered when investigating the performance regression for #2643, but it appears the gain is independent of that pull request, so this fix should be pushed to master independently, and #2643 should be investigated further.

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Review checklist

The author performs the actions on the checklist. The reviewer evaluates the work and checks the boxes as they are completed.

• Summary. Write a summary of the changes. Explain what you did to fix the issue, and why you did it. Present the changes in a logical order. Instead of writing a summary in the pull request, you may push a clean Git history.
• Documentation. Write documentation for new functions. Update documentation for functions that changed, or complete documentation where it is missing.
• Tests. Write unit tests for every change. Write the unit tests that were missing before the changes. Include any examples from the reported issue as integration tests.
• Clean up. The changes are already clean. Clean up anything near the changes that you noticed while working. This does not mean only spatially near the changes, but logically near: any code that interacts with the changes!

[github-actions]

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test/regression-evm/test-straight-line-no-invalid.sh	-0.024035	0
test/regression-evm/test-branching-invalid.sh	-0.038732	0
test/regression-evm/test-pop1.sh	-0.023056	0
test/regression-evm/test-straight-line.sh	-0.025973	-0.005669
test/regression-evm/test-sumTo10.sh	-0.096893	0.002719
test/regression-wasm/test-locals.sh	-0.000227	-0.000002
test/regression-wasm/test-simple-arithmetic.sh	-0.001085	0
test/regression-wasm/test-wrc20.sh	-0.003158	0.000960
test/regression-wasm/test-loops.sh	-0.002920	0
test/regression-wasm/test-memory.sh	-0.002254	0