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.

Author: chrahunt

src/pip/_internal/req/req_install.py

@@ -371,7 +371,6 @@ def ensure_build_location(self, build_dir):
         if not os.path.exists(build_dir):
             logger.debug('Creating directory %s', build_dir)
             os.makedirs(build_dir)
-            write_delete_marker_file(build_dir)
         return os.path.join(build_dir, name)
 
     def _set_requirement(self):