Move title tag cleanup to page-title-list service

[mydea]

When using engines, each engine might run the instance initializer, leading to the title tag being removed. This is related to #112, but also to ember-fastboot/ember-cli-head#41.

This PR removes the instance initializer, and instead removes any existing title tags in the service's init() method.

Since the service is shared between engines, this will only be run once.

[mydea]

If ember-fastboot/ember-cli-head#45 is merged & released, an even nicer solution would be to overwrite the head-layout component, and do the tearing down of the title in it's init() method. This would ensure that the app is never without a title. But this could be done at a later time, I guess. This would also address #116.

[tim-evans]

Thanks for the PR, @mydea ! This looks good to me- I believe the test suite already covers for this. I'll verify on my end 👍

[tim-evans]

Released in 4.0.2