Remove duplicate function dropColumns

Since support for func_get_args was added to the dropColumn function, the dropColumns function is nothing more than a duplicate.

Signed-off-by: Dries Vints <dries.vints@gmail.com>
Add new unit tests for dropColumn function

Signed-off-by: Dries Vints <dries.vints@gmail.com>
Merge pull request #977 from driesvints/dv/remove-duplicate-removeColumns-function

Remove duplicate dropColumns function