Deprecate, and remove ENV vars from bin/rspec-stackprof

The following discussion from !81750 (merged) should be addressed:

• @tkuah started a discussion:

  Because of the inability to mix options for stackprof and rspec, we used ENV vars. I think if this is successful, we should deprecate, and remove usage of ENV vars

Implementation plan

1. Remove support for ENV vars for the following from bin/rspec-stackprof (as there is already support via command line flags like --mode:
   • MODE
   • INTERVAL
   • LIMIT
   • RAW
   • OUTPUT
   • SPEEDSCOPE
2. Update documentation where these ENV vars are referenced, like in doc/development/performance.md