Fix config schema [#3567939]

Problem/Motivation

The config schema declaration is slightly incorrect so that in effect, the cdn_hostname does not get recognized. This leads to a warning during installation:

[warning] Schema errors for bunny_optimizer.settings with the following errors: bunny_optimizer.settings:cdn_hostname missing schema. These errors mean there is configuration that does not comply with its schema. This is not a fatal error, but it is recommended to fix these issues. For more information on configuration schemas, check out the documentation.

Steps to reproduce

Install a profile with this module as part of it.

Proposed resolution

Fix the config schema. The problem is that there is an unnecessary level of nesting in the schema. Simply removing that fixes the warning.

Remaining tasks

User interface changes

API changes

Data model changes

Issue fork bunny_optimizer-3567939

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3567939-fix-config-schema changes, plain diff MR !10
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