Temporarily remove exoplayer

[uvjustin]

We proposed to change the format of the ExternalApp call to ExoPlayer for the Android App. Since the app has already been released with the old format of the call, we will wait until an app with the new format of the call has been released before allowing ExoPlayer to be used in the frontend.
See also:
#7010
home-assistant/android#932

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example configuration

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io