Remove select_for_update option for FieldDataCache

We've already been ignoring the param for a long time. This just
removes it entirely.

lms/djangoapps/courseware/model_data.py

@@ -687,7 +687,7 @@ class FieldDataCache(object):
     A cache of django model objects needed to supply the data
     for a module and its descendants
     """
-    def __init__(self, descriptors, course_id, user, select_for_update=False, asides=None, read_only=False):
+    def __init__(self, descriptors, course_id, user, asides=None, read_only=False):
         """
         Find any courseware.models objects that are needed by any descriptor
         in descriptors. Attempts to minimize the number of queries to the database.
@@ -698,7 +698,6 @@ class FieldDataCache(object):
         descriptors: A list of XModuleDescriptors.
         course_id: The id of the current course
         user: The user for which to cache data
-        select_for_update: Ignored
         asides: The list of aside types to load, or None to prefetch no asides.
         read_only: We should not perform writes (they become a no-op).
         """
@@ -785,7 +784,7 @@ class FieldDataCache(object):
     @classmethod
     def cache_for_descriptor_descendents(cls, course_id, user, descriptor, depth=None,
                                          descriptor_filter=lambda descriptor: True,
-                                         select_for_update=False, asides=None, read_only=False):
+                                         asides=None, read_only=False):
         """
         course_id: the course in the context of which we want StudentModules.
         user: the django user for whom to load modules.
@@ -794,9 +793,8 @@ class FieldDataCache(object):
             the supplied descriptor. If depth is None, load all descendant StudentModules
         descriptor_filter is a function that accepts a descriptor and return whether the field data
             should be cached
-        select_for_update: Ignored
         """
-        cache = FieldDataCache([], course_id, user, select_for_update, asides=asides, read_only=read_only)
+        cache = FieldDataCache([], course_id, user, asides=asides, read_only=read_only)
         cache.add_descriptor_descendents(descriptor, depth, descriptor_filter)
         return cache