PYTHON-5439 Remove dead link in PyMongo 4 migration guide (mongodb#2428)

Author: Steven Silvester

doc/migrate-to-pymongo4.rst

@@ -837,7 +837,6 @@ pymongo.GEOHAYSTACK is removed
 
 Removed :attr:`pymongo.GEOHAYSTACK`. Replace with "geoHaystack" or create a
 2d index and use $geoNear or $geoWithin instead.
-See https://dochub.mongodb.org/core/4.4-deprecate-geoHaystack.
 
 UUIDLegacy is removed
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