Simplify "Preparing metadata" logging

Do not mention wheel, as this code path is also used
in legacy editable mode with isolation, where
no wheel is involved.

This also aligns with the new log entry for setup.py egg-info
which says "Preparing metadata (setup.py)".

src/pip/_internal/operations/build/metadata.py

@@ -23,9 +23,7 @@ def generate_metadata(build_env: BuildEnvironment, backend: Pep517HookCaller) ->
         # Note that Pep517HookCaller implements a fallback for
         # prepare_metadata_for_build_wheel, so we don't have to
         # consider the possibility that this hook doesn't exist.
-        runner = runner_with_spinner_message(
-            "Preparing wheel metadata (pyproject.toml)"
-        )
+        runner = runner_with_spinner_message("Preparing metadata (pyproject.toml)")
         with backend.subprocess_runner(runner):
             distinfo_dir = backend.prepare_metadata_for_build_wheel(metadata_dir)