[llvm] [Dexter] Remove outdated imp dependency (PR #111833)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `cross-project-tests-sie-ubuntu` running on `doug-worker-1a` while building `cross-project-tests` at step 6 "test-build-unified-tree-check-cross-project".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/181/builds/6664

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-cross-project) failure: test (failure)
******************** TEST 'cross-project-tests :: debuginfo-tests/dexter/feature_tests/subtools/list-debuggers/list-debuggers.test' FAILED ********************
Exit Code: 2

Command Output (stderr):
--
RUN: at line 4: "/usr/bin/python3.8" "/home/buildbot/buildbot-root/cross-project-tests-sie-ubuntu/llvm-project/cross-project-tests/debuginfo-tests/dexter/dexter.py" list-debuggers | /home/buildbot/buildbot-root/cross-project-tests-sie-ubuntu/build/bin/FileCheck /home/buildbot/buildbot-root/cross-project-tests-sie-ubuntu/llvm-project/cross-project-tests/debuginfo-tests/dexter/feature_tests/subtools/list-debuggers/list-debuggers.test
+ /usr/bin/python3.8 /home/buildbot/buildbot-root/cross-project-tests-sie-ubuntu/llvm-project/cross-project-tests/debuginfo-tests/dexter/dexter.py list-debuggers
+ /home/buildbot/buildbot-root/cross-project-tests-sie-ubuntu/build/bin/FileCheck /home/buildbot/buildbot-root/cross-project-tests-sie-ubuntu/llvm-project/cross-project-tests/debuginfo-tests/dexter/feature_tests/subtools/list-debuggers/list-debuggers.test


****************************************


```

</details>

https://github.com/llvm/llvm-project/pull/111833