[FIX] MongoError during startup saying "ns not found"

[sampaiodiego]

Proposed changes (including videos or screenshots)

During a mutli-instance startup, if there is more than one instance being started at the same time, they will all try to execute the same drop command, but just a single one would succeed and the other ones would show the error below:

=== UnHandledPromiseRejection ===
MongoError: ns not found
    at MessageStream.messageHandler (/.../.meteor/packages/npm-mongo/.3.9.1.gjvjg.whtjk6++os+web.browser+web.browser.legacy+web.cordova/npm/node_modules/mongodb/lib/cmap/connection.js:272:20)
    at MessageStream.emit (events.js:314:20)
    at MessageStream.EventEmitter.emit (domain.js:483:12)
    at processIncomingData (/.../.meteor/packages/npm-mongo/.3.9.1.gjvjg.whtjk6++os+web.browser+web.browser.legacy+web.cordova/npm/node_modules/mongodb/lib/cmap/message_stream.js:144:12)
...

So instead of dropping the collection, I'm changing to just remove the content of the collection.

Issue(s)

Closes #15485

Steps to test or reproduce

Further comments