arg from the current FExpr.
FExpr represents a collection of columns, or a columnset. Some
of those columns are computed while others are specified “by reference”,
f[int]. This method allows you to
remove by-reference columns from an existing FExpr.
The columns to remove. These must be “columns-by-reference”, i.e. they cannot be computed columns.
New FExpr which is a obtained from the current FExpr by removing
the columns in