Fix cancelCallback for child_added events

[schmidt-sebastian]

Fixes #4818

This simplifies the code in ChildEventRegistration. The class is meant to support multiple callbacks, but only one cancel callback. This broke in our refactor. Luckily, the class is never used for more than callback, so I was able to simplify the code and also fix the bug (which was due to the fact that the cancel callback was never registered under 'cancel').

[changeset-bot]

🦋 Changeset detected

Latest commit: 4c804a6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 7 packages

Name	Type
@firebase/database	Patch
firebase	Patch
@firebase/rules-unit-testing	Patch

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[google-oss-bot]

Binary Size Report

Affected SDKs

• @firebase/database

  Type	Base (d47fb41)	Head (31f6dca)	Diff
  browser	291 kB	291 kB	-525 B (-0.2%)
  esm2017	261 kB	261 kB	-499 B (-0.2%)
  main	294 kB	293 kB	-530 B (-0.2%)
  module	291 kB	291 kB	-525 B (-0.2%)

• @firebase/database-exp

  Type	Base (d47fb41)	Head (31f6dca)	Diff
  browser	242 kB	242 kB	-499 B (-0.2%)
  main	273 kB	273 kB	-530 B (-0.2%)
  module	242 kB	242 kB	-499 B (-0.2%)

• firebase

  Type	Base (d47fb41)	Head (31f6dca)	Diff
  firebase-database.js	183 kB	183 kB	-258 B (-0.1%)
  firebase.js	869 kB	869 kB	-259 B (-0.0%)

Test Logs

• Base (d47fb414): https://github.com/firebase/firebase-js-sdk/actions/runs/779289973
• Head (31f6dcab): https://github.com/firebase/firebase-js-sdk/actions/runs/779548924

[google-oss-bot]

Size Analysis Report

Affected Products

Diffs between base commit (d47fb41) and head commit (31f6dca) are too large (479,509 characters) to display.

Please check below links to see details from the original test log.

Test Logs

• Head (31f6dca): https://github.com/firebase/firebase-js-sdk/actions/runs/779548924