PYTHON-1301/PYTHON-1302/PYTHON-1588 Remove deprecated cursor manager APIs

[ShaneHarvey]

This change implements:

• PYTHON-1301 Remove support for cursor managers
• PYTHON-1302 Remove MongoClient.kill_cursors
• PYTHON-1588 Remove MongoClient.close_cursor

[ShaneHarvey]

The following test is failing on MongoDB 4.0+:

 [2021/01/15 23:50:13.857] ERROR [2.146s]: test_break_resume_token (test_custom_types.TestClusterChangeStreamsWCustomTypes)
 [2021/01/15 23:50:13.857] ----------------------------------------------------------------------
 [2021/01/15 23:50:13.857] Traceback (most recent call last):
 [2021/01/15 23:50:13.857]   File "/data/mci/07146fdd00f40c8dc2f3160548b8d5c8/src/test/test_custom_types.py", line 826, in test_break_resume_token
 [2021/01/15 23:50:13.857]     self.insert_and_check(change_stream, docs[0], docs[0])
 [2021/01/15 23:50:13.857]   File "/data/mci/07146fdd00f40c8dc2f3160548b8d5c8/src/test/test_custom_types.py", line 751, in insert_and_check
 [2021/01/15 23:50:13.857]     self.input_target.insert_one(insert_doc)
 [2021/01/15 23:50:13.857]   File "/data/mci/07146fdd00f40c8dc2f3160548b8d5c8/src/pymongo/collection.py", line 701, in insert_one
 [2021/01/15 23:50:13.857]     self._insert(document,
 [2021/01/15 23:50:13.857]   File "/data/mci/07146fdd00f40c8dc2f3160548b8d5c8/src/pymongo/collection.py", line 616, in _insert
 [2021/01/15 23:50:13.857]     return self._insert_one(
 [2021/01/15 23:50:13.857]   File "/data/mci/07146fdd00f40c8dc2f3160548b8d5c8/src/pymongo/collection.py", line 605, in _insert_one
 [2021/01/15 23:50:13.857]     self.__database.client._retryable_write(
 [2021/01/15 23:50:13.857]   File "/data/mci/07146fdd00f40c8dc2f3160548b8d5c8/src/pymongo/mongo_client.py", line 1478, in _retryable_write
 [2021/01/15 23:50:13.857]     return self._retry_with_session(retryable, func, s, None)
 [2021/01/15 23:50:13.857]   File "/data/mci/07146fdd00f40c8dc2f3160548b8d5c8/src/pymongo/mongo_client.py", line 1364, in _retry_with_session
 [2021/01/15 23:50:13.857]     return self._retry_internal(retryable, func, session, bulk)
 [2021/01/15 23:50:13.857]   File "/data/mci/07146fdd00f40c8dc2f3160548b8d5c8/src/pymongo/mongo_client.py", line 1396, in _retry_internal
 [2021/01/15 23:50:13.857]     return func(session, sock_info, retryable)
 [2021/01/15 23:50:13.857]   File "/data/mci/07146fdd00f40c8dc2f3160548b8d5c8/src/pymongo/collection.py", line 603, in _insert_command
 [2021/01/15 23:50:13.857]     _check_write_command_response(result)
 [2021/01/15 23:50:13.857]   File "/data/mci/07146fdd00f40c8dc2f3160548b8d5c8/src/pymongo/helpers.py", line 226, in _check_write_command_response
 [2021/01/15 23:50:13.857]     _raise_last_write_error(write_errors)
 [2021/01/15 23:50:13.857]   File "/data/mci/07146fdd00f40c8dc2f3160548b8d5c8/src/pymongo/helpers.py", line 207, in _raise_last_write_error
 [2021/01/15 23:50:13.857]     raise DuplicateKeyError(error.get("errmsg"), 11000, error)
 [2021/01/15 23:50:13.857] pymongo.errors.DuplicateKeyError: E11000 duplicate key error collection: pymongo_test.test index: _id_ dup key: { _id: 1 }, full error: {'index': 0, 'code': 11000, 'codeName': 'DuplicateKey', 'errmsg': 'E11000 duplicate key error collection: pymongo_test.test index: _id_ dup key: { _id: 1 }'}

Not really sure why yet. Perhaps some tests are mistakenly not dropping the collection/database?

[ShaneHarvey]

Rebased to fix merge conflict in the docs.