Remove PRMS6_development-specific code

The pyPRMS library has specific code to handle control file variables, parameters, and output file types from the original PRMS6_development code, which was a rewrite from PRMS5 code (https://github.com/EC-USGS/prms6). The official PRMS software repo (https://github.com/nhm-usgs/prms) has now released a PRMS version 6.0.0 (branch: 5.2.2.2 commit: 50f289b) that is based on the original PRMS5 codebase.

This code will need to be removed and/or modified so the original prms6 handling does not collide with the PRMS 6.0.0 requirements.