refactor: remove explicit generics in rxjs usages

[crisbeto]

Removes all the places where we set generic explicitly for rxjs objects, in order to avoid potential issues with rxjs 6.4.

Fixes #15307.

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "branch-manager" is failing

If you want your PR to be merged, it has to pass all the CI checks.

If you can't get the PR to a green state due to flakes or broken master, please try rebasing to master and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}