delete nonuniquekey() method

it's not used and it is no loner faster than nonunique()

Author: alyst

src/abstractdataframe/abstractdataframe.jl

@@ -621,17 +621,6 @@ unique!(df)  # modifies df
 """
 (unique, unique!)
 
-function nonuniquekey(df::AbstractDataFrame)
-    # Here's another (probably a lot faster) way to do `nonunique`
-    # by grouping on all columns. It will fail if columns cannot be
-    # made into PooledDataVector's.
-    gd = groupby(df, _names(df))
-    idx = [1:length(gd.idx)][gd.idx][gd.starts]
-    res = fill(true, nrow(df))
-    res[idx] = false
-    res
-end
-
 # Count the number of missing values in every column of an AbstractDataFrame.
 function colmissing(df::AbstractDataFrame) # -> Vector{Int}
     nrows, ncols = size(df)