DOCSP-40217 -- Remove reference to shared tier clusters in indexes limitations

[jvincent-mongodb]

DESCRIPTION

Remove reference to shared tier clusters in indexes limitations.

STAGING

https://preview-mongodbjvincentmongodb.gatsbyjs.io/compass/DOCSP-40217/indexes/#limitations

JIRA

https://jira.mongodb.org/browse/DOCSP-40217

BUILD LOG

https://workerpool-boxgs.mongodbstitch.com/pages/job.html?collName=queue&jobId=667f271dfe162e50062b3e9a

Self-Review Checklist

• [X] Is this free of any warnings or errors in the RST?
• [X] Is this free of spelling errors?
• [X] Is this free of grammatical errors?
• [X] Is this free of staging / rendering issues?
• [X] Are all the links working?

External Review Requirements

What's expected of an external reviewer?