Remove unnecessary InstallRequirement cleanup in install_given_reqs

install_given_reqs is only called from InstallCommand.run, which calls
RequirementSet.cleanup_files, which calls
InstallRequirement.remove_temporary_source for each InstallRequirement,
so the call here was not necessary.

We have test coverage affirming this still works as expected in
tests/functional/test_install_cleanup.py.

src/pip/_internal/req/__init__.py

@@ -86,7 +86,6 @@ def install_given_reqs(
                 )
                 if should_commit:
                     uninstalled_pathset.commit()
-            requirement.remove_temporary_source()
 
             installed.append(InstallationResult(requirement.name))