apacheGH-38323: [CI][Python] Use system gdb on test-conda-python

gdb on conda-forge frequently conflicts with the newest Python version
(example for Python 3.12: conda-forge/gdb-feedstock#66)

We needn't install gdb from conda-forge, though, as the Python interpreter used for gdb is entirely independent from the Python interpreter under test.

ci/docker/conda-python.dockerfile

@@ -25,7 +25,6 @@ COPY ci/conda_env_python.txt \
      /arrow/ci/
 RUN mamba install -q -y \
         --file arrow/ci/conda_env_python.txt \
-        gdb \
         python=${python} \
         nomkl && \
     mamba clean --all