[SPARKR][MINOR] Remove reference link for common Windows environment variables

## What changes were proposed in this pull request?

The PR removes reference link in the doc for environment variables for common Windows folders. The cran check gave code 503: service unavailable on the original link.

## How was this patch tested?

Manual check.

Author: Junyang Qian <junyangq@databricks.com>

Closes #14767 from junyangq/SPARKR-RemoveLink.