Review and remove dead code

[Michael Stackhouse]

There are several sections of code that aren't truly necessary (or never were necessary) anymore that should be dropped.

• Tplyr/R/table.R

  Line 45 in 6876054

   if(missing(target)){ 

• Tplyr/R/layer.R

  Line 106 in 6876054

   as_tplyr_layer.tplyr_subgroup_layer <- function(parent, target_var, by, where, type, ...) { 

  <- this concept doesn't actually exist

Other untested lines of code should be re-evaluated based on code coverage.

[Michael Stackhouse]

Closed via #167