Remove forgotten TODO (#1735)

This TODO was resolved in: https://github.com/PyCQA/pylint/pull/1733#pullrequestreview-75930778

Resolves: #1301

pylint/checkers/stdlib.py

@@ -107,7 +107,7 @@ class StdlibChecker(BaseChecker):
                   'The warning is emitted when a threading.Thread class '
                   'is instantiated without the target function being passed. '
                   'By default, the first parameter is the group param, not the target param. '),
-        'W1507': ('Using copy.copy(os.environ). Use os.environ.copy() '  # TODO: number
+        'W1507': ('Using copy.copy(os.environ). Use os.environ.copy() '
                   'instead. ',
                   'shallow-copy-environ',
                   'os.environ is not a dict object but proxy object, so '