Drop primary key if its column is dropped

[Christopher Kramer]

When dropping a column which is (part of) a primary key, the primary key should get dropped implicitly. Same for UNIQUE table-constraints. This already works if the definition was done through a column-constraint rather than a table-constraint. So in other words, this usually already works for primary keys if they are not compound primary keys.

This was originally mentioned in issue #12 (https://bitbucket.org/phpliteadmin/public/issues/12/cannot-re-edit-table-remove-rename-columns#comment-19341459).