Missing implementation of drop_column for SQLite
Some migration code are based on the assumption that the column will be dropped
when the code is run a second times.
The drop_column [1] should be implemented the same way as column_rename [2]
[1]
http://hg.tryton.org/trytond/file/9a2e382df2f8/trytond/backend/sqlite/table.py#l301
[2]
http://hg.tryton.org/trytond/file/9a2e382df2f8/trytond/backend/sqlite/table.py#l91