[BUGFIX] migration of datasource selector when it's a datasource var ref

[celian-garcia]

Description

When datasource selector contains a reference to a datasource variable, the syntax is "datasource": "${datasource}", and not the current migration result:

"datasource": {
      "kind": "PrometheusDatasource",
      "name": "${datasource}"
    },

This PR fixes that problem and remove an unused variable.

---

Note that we need this PR merged perses/shared#242 and a shared package update , so it will also support the ${} syntax and the test will pass.

This is why the tests are failing

Screenshots

Checklist

• Pull request has a descriptive title and context useful to a reviewer.
• Pull request title follows the [<catalog_entry>] <commit message> naming convention using one of the
  following catalog_entry values: FEATURE, ENHANCEMENT, BUGFIX, BREAKINGCHANGE, DOC,IGNORE.
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