cleaning out unnecessary nga-sub flavors

...gov/usgs/earthquake/nshmp/gmm/AbrahamsonGulerce_2020.java

src/main/java/gov/usgs/earthquake/nshmp/gmm/Gmm.java

...in/java/gov/usgs/earthquake/nshmp/gmm/KuehnEtAl_2020.java

...n/java/gov/usgs/earthquake/nshmp/gmm/ParkerEtAl_2020.java

src/test/java/gov/usgs/earthquake/nshmp/gmm/NgaSubAG20.java

...t/java/gov/usgs/earthquake/nshmp/gmm/NgaSubInterface.java

src/test/java/gov/usgs/earthquake/nshmp/gmm/NgaSubSlab.java

src/test/resources/gmm/NGA_SUB_AG20_results.csv

src/test/resources/gmm/NGA_SUB_interface_results.csv

src/test/resources/gmm/NGA_SUB_slab_results.csv