fix: Reverting change to include dependency of mlflow-export-import to unblock pypi releases (#775)

## Description
https://github.com/ecmwf/anemoi-core/actions/runs/20342675367 - Current
pypi releases are blocked because of
https://github.com/ecmwf/anemoi-core/pull/748/changes where
'mlflow-export-import' was added to the pyproject.toml as a dependency.

Because the package is poorly maintained and don't have releases we used
the git+https. According to https://til.dchan.cc/posts/11-29-2022/ and
the error, this blocks the release to pypi.

## What problem does this change solve?
(https://github.com/ecmwf/anemoi-core/actions/runs/20342675367)

## What issue or task does this change relate to?
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##  Additional notes ##
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(cherry picked from commit f7d2acc)

Author: anaprietonem

training/pyproject.toml

@@ -79,9 +79,6 @@ optional-dependencies.docs = [
   "sphinx-argparse",
   "sphinx-rtd-theme",
 ]
-optional-dependencies.mlflowsync = [
-  "mlflow-export-import @ git+https://github.com/mlflow/mlflow-export-import.git@master",
-]
 optional-dependencies.plotting = [
   "distinctipy>=1",
 ]

training/src/anemoi/training/utils/mlflow_sync.py

@@ -73,7 +73,7 @@ def close_and_clean_temp(server2server: str, artifact_path: Path) -> None:
 except ImportError:
     msg = (
         "The 'mlflow-export-import' package is not installed."
-        "You can install it doing pip install anemoi-training[mlflowsync]"
+        "Please install it from https://github.com/mlflow/mlflow-export-import"
     )
     raise ImportError(msg) from None