removed unused zhao16 and mcverry

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

.../java/gov/usgs/earthquake/nshmp/gmm/McVerryEtAl_2000.java

...ain/java/gov/usgs/earthquake/nshmp/gmm/ZhaoEtAl_2006.java

...ain/java/gov/usgs/earthquake/nshmp/gmm/ZhaoEtAl_2016.java

src/main/resources/gmm/coeffs/McVerry00_gm.csv

src/main/resources/gmm/coeffs/McVerry00_mh.csv

src/main/resources/gmm/coeffs/Zhao16_crust.csv

src/main/resources/gmm/coeffs/Zhao16_interface.csv

src/main/resources/gmm/coeffs/Zhao16_siteamp.csv

src/main/resources/gmm/coeffs/Zhao16_slab.csv