common: Remove probably superfluous import

The purpose of this import is unclear.

This import caused the following error with Sphinx 3.0:

Extension error:
Could not import extension rtemsdomain (exception: cannot import name 'l_')

common/rtemsdomain.py

@@ -3,7 +3,6 @@ from docutils.parsers.rst import directives
 
 from sphinx import addnodes
 from sphinx.roles import XRefRole
-from sphinx.locale import l_, _
 from sphinx.directives import ObjectDescription
 from sphinx.domains import Domain, ObjType, Index
 from sphinx.util.nodes import make_refnode