Bug 1708260: Remove "imp" usage when loading Mach r=firefox-build-system-reviewers,glandium

We always run on Python 3.6+, so the `imp` usage was dead code.

Differential Revision: https://phabricator.services.mozilla.com/D120398

mach

@@ -125,16 +125,12 @@ from __future__ import absolute_import, print_function, unicode_literals
 import os
 import sys
 
-def load_mach(dir_path, mach_path):
-    if sys.version_info < (3, 5):
-        import imp
-        mach_bootstrap = imp.load_source('mach_bootstrap', mach_path)
-    else:
-        import importlib.util
-        spec = importlib.util.spec_from_file_location('mach_bootstrap', mach_path)
-        mach_bootstrap = importlib.util.module_from_spec(spec)
-        spec.loader.exec_module(mach_bootstrap)
 
+def load_mach(dir_path, mach_path):
+    import importlib.util
+    spec = importlib.util.spec_from_file_location('mach_bootstrap', mach_path)
+    mach_bootstrap = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(mach_bootstrap)
     return mach_bootstrap.bootstrap(dir_path)