Remove default "g1" group from Tissue Specificty output

[ChristopherWilks]

While it may be needed internally, it could be confusing to the user what the "group" column with value "g1" represents.

To reproduce:

inclusion_group1<-inclusion_group1$from_url("http://snaptron.cs.jhu.edu/gtex/snaptron?regions=chr4:20763023-20763023&either=2&rfilter=strand:-")
inclusion_group2 <- SnaptronQueryBuilder$new()
inclusion_group2<-inclusion_group2$from_url("http://snaptron.cs.jhu.edu/gtex/snaptron?regions=chr4:20763098-20763098&either=1&rfilter=strand:-")

ts<-tissue_specificity(list(inclusion_group1, inclusion_group2))