Fix #2685.
Ensure Table.remove_columns works when given a single column name
Merge pull request #2685 from mdboom/votable/name-error

Bug: votable.conesearch fails with `NameError: global name 'votable' is not defined`
Merge pull request #2699 from mhvk/table-remove-columns-single-column

Table.remove_columns on a string