Retire GNU build starting on June 1st, 2022

[aliabdolali]

Is your feature request related to a problem? Please describe.
With CMake build system, the existing GNU build of WW3 can be retired, which makes code maintenance and development easier. It is announced at the WW3 developer meeting on April 19th. The trusted institutions agreed with the plan as well.

Describe the solution you'd like
The gnu build should be removed from the repo including model/bin/w3_make, model/bin/w3_automake, regtests/bin/run_test and matrix_<institution>.

[JessicaMeixner-NOAA]

At this point, the GNU build is no longer supported, however it has not been fully removed from the code (see PR #726) as the wiki page and manual still need to be updated. These updates are a work in progress.