feat(NODE-4184): do not throw with explain and write conceern

[durran]

Description

The driver will no longer throw an error if aggregate is used with explain and write concern. The server simply ignores the write concern in this case.

What is changing?

• Removes the check and throw from the aggregation operation.
• Adds a unit test to protect from regressions.

Is there new documentation needed for these changes?

None

What is the motivation for this change?

NODE-4184

Release Highlight

Explain + Write Concern on an aggregation no longer throws.

The server will silently ignore the write concern.

// Previously this would throw a MongoInvalidArgumentError, now it will return explain results.
collection.aggregate([{ $match: { n: 1 }}], { writeConcern: { w: 1 }, explain: 'verbose' });

Double check the following

• Ran npm run check:lint script
• Self-review completed using the steps outlined here
• PR title follows the correct format: type(NODE-xxxx)[!]: description
  • Example: feat(NODE-1234)!: rewriting everything in coffeescript
• Changes are covered by tests
• New TODOs have a related JIRA ticket