Remove injections initializer

[gorner]

One of the changes in PR #49 was (emphasis added):

• Updated code to explicitly inject the pusher service where necessary due to the removal of implicit injection support in 4.x (though leaving the initializer itself appears to be OK for now)

However, on Ember 4, such injection initializers raise a deprecation warning, and on Ember 5, they result in the error TypeError: application.inject is not a function which will prevent the app from running. As such, the injection initializer must be completely removed in order for ember-pusher to work on Ember 5.

This is a potentially breaking change for any users of this addon on Ember versions prior to 4.x that have not already resolved the 3.x implicit-injections deprecation introduced in Ember 3.26 (which warned app developers to add explicit injections for any services they used). However, as the only release of ember-pusher 3.x so far has been an alpha release, I don't believe a further major version bump is required.