Fix error for stt when async_get_engine is not implemented

[jbouwh]

Proposed change

Fix error for stt when async_get_engine is not implemented.

When the demo platform is set up. The following error is logged:

2023-06-28 08:04:55.746 ERROR (MainThread) [homeassistant.components.stt.legacy] Error setting up platform: demo
Traceback (most recent call last):
  File "/workspaces/core/homeassistant/components/stt/legacy.py", line 68, in async_setup_platform
    provider = await platform.async_get_engine(hass, p_config, discovery_info)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: module 'homeassistant.components.demo.stt' has no attribute 'async_get_engine'

This PR fixes this by checking removing the legacy stt platform from the demo integration.

Type of change

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

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.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• I have followed the perfect PR recommendations
• The code has been formatted using Black (black --fast homeassistant tests)
• 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

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.
• Untested files have been added to .coveragerc.

To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[home-assistant]

Hey there @home-assistant/core, @pvizeli, mind taking a look at this pull request as it has been labeled with an integration (stt) you are listed as a code owner for? Thanks!

Code owner commands

Code owners of stt can trigger bot actions by commenting:

• @home-assistant close Closes the pull request.
• @home-assistant rename Awesome new title Renames the pull request.
• @home-assistant reopen Reopen the pull request.
• @home-assistant unassign stt Removes the current integration label and assignees on the pull request, add the integration domain after the command.

[emontnemery]

I think this should be fixed in the demo integration, not in stt, by removing this line

core/homeassistant/components/demo/__init__.py

Line 53 in 0bfb81e

Platform.STT,

This should have been done in #94585, but was forgotten