写一段python代码，调用pymongo库中的方法 update_many删除mongodb数据库名字为db1的，名为table11的collections中的字段c1

from pymongo import MongoClient

# connect to the MongoDB instance
client = MongoClient('localhost', 27017)

# access the database named "db1"
db = client.db1

# access the collection named "table11"
collection = db.table11

# delete the field "c1" from the collection
collection.update_many({}, {'$unset': {'c1': 1}})