Pyroscope: Remove support for old pyroscope

[aocenas]

What is this feature?

Removes support for old pyroscope backend and relevant client code. Also removes the backend type switch from the config page as it is not needed anymore.

Why do we need this feature?

For one there was a bit of confusion around the backend type selection and as we have Grafana Pyroscope v1 release the old pyroscope support is less relevant.

Who is this feature for?

[Add information on what kind of user the feature is for.]

Which issue(s) does this PR fix?:

Fixes #

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[joey-grafana]

• Will need to update the text here
  

• And in the docs
  

• Running make devenv sources=phlare is fine, can connect to Phlare and query data. Running make devenv sources=pyroscope results in the following:
  

[aocenas]

@joey-grafana thanks for noticing the docs and devenv stuff, should be all fixed now.

[petethepig]

Curious about this. Why remove query, start and end? Is it because we were not actually using them on the frontend?

[aocenas]

Because the phlare client (which is also new pyroscope client) does not use it so we would just have unused args there. We actually send it from frontend so it will be easy to add them again once pyroscope supports it.