Remove unnecessary write_delete_marker_file
Remove unnecessary write_delete_marker_file

InstallRequirement only checks for a delete marker file in source_dir.
Since the result of ensure_build_location is set to source_dir (in
ensure_has_source_dir), and ensure_build_location returns a subdirectory
of the build_directory to which we write the delete marker file, then
this delete marker file is never used.

sbidoul/pip,pypa/pip,pfmoore/pip,sbidoul/pip,pradyunsg/pip,pfmoore/pip,pypa/pip,pradyunsg/pip