Executor: Remove duplicate entry with dry-run argument (#4660)

# Pull Request Check List

Resolves: #3097 

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- [x] Added **tests** for changed code. I adapted the test in the `tests/console/commands/plugin/test_remove.py` file to reflect the bug fix.
- [ ] Updated **documentation** for changed code.

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Apart from running the ci linting steps and pytest locally for both python 3.6 and 3.8, I also tested the code by running
```
docker run --rm -i --entrypoint bash python:3.8 <<EOF
set -e
python -m pip install -q git+https://github.com/mmacchia/poetry.git@issue/3097


python -m poetry new foobar
pushd foobar
sed -i /pytest/d pyproject.toml
python -m poetry add --dry-run pycowsay
python -m poetry run pip list
EOF
```
The corresponding output is:
```
Created package foobar in foobar
/foobar /
Creating virtualenv foobar-lWDpn5M1-py3.8 in /root/.cache/pypoetry/virtualenvs
Using version ^0.0.0.1 for pycowsay

Updating dependencies
Resolving dependencies...

Writing lock file

Package operations: 1 install, 0 updates, 0 removals

  • Installing pycowsay (0.0.0.1)
Package    Version
---------- -------
pip        21.2.4
setuptools 58.1.0
wheel      0.37.0
```
As expected, the command `poetry add --dry-run pycowsay` yielded a single output (` • Installing pycowsay (0.0.0.1)`) and did not add the package to the project (see the output of `poetry run pip list`).

The change in the PR solves the duplication issue because the creation of the console output when `not self._enabled` is taken care of by the boolean output of the`self._should_write_operation(operation)` function.

Please feel free to get in touch if I can help adding more test cases for the `--dry-run` argument or the `_enabled` flag and to add any feedback. I would be happy to improve/clarify the code in this PR.

Co-authored-by: Marco Macchia <mmacchia@wayfair.com>