fix: remove ContentDocumentLink from restricted sfdc objects (#69)

ContentDocumentLink cannot be queried without a where clause by default;
however, this setting can be changed (see [Salesforce
docs](https://developer.salesforce.com/docs/atlas.en-us.object_reference.meta/object_reference/sforce_api_objects_contentdocumentlink.htm)).

I propose removing this object from the restricted Salesforce Objects
list so that it can be synced with this tap.

Closes #70.

tap_salesforce/salesforce/__init__.py

@@ -78,7 +78,6 @@
 # The following objects have certain WHERE clause restrictions so we exclude them.
 QUERY_RESTRICTED_SALESFORCE_OBJECTS = {
     "Announcement",
-    "ContentDocumentLink",
     "CollaborationGroupRecord",
     "Vote",
     "IdeaComment",