Remove plr for postgis

[benjaminjb]

Checklist:

• [x] Have you added an explanation of what your changes do and why you'd like them to be included?
• [x] Have you updated or added documentation for the change, as applicable?
• [x] Have you tested your changes on all related environments with successful results, as applicable?

Type of Changes:

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

What is the current behavior? (link to any open issues here)

PL/R is not included in the Dockerfile (as of https://github.com/CrunchyData/crunchy-containers/commit/d823c02e271ea1bfd24645efe6d3c153dbaf2361#diff-2bd850e914f86acf4c6087084b39b158a531b5f11a8f9e98d9b2a5c65f6586b3L27 ), so this removes it from the setup.sql and docs for PostGIS

What is the new behavior (if this is a feature change)?

Removes possible error and point of confusion.

Other information: [sc-12487]