removing all column metadata via CLI

[Radim Kolar]

I cant find way how to remove all columns definitions without CF 
import/export.

[default@int4] update column family sipdb with column_metadata = [];
Syntax error at position 51: required (...)+ loop did not match anything 
at input ']'

[default@int4] update column family sipdb with column_metadata = [{}];
Command not found: `update column family sipdb with column_metadata = 
[{}];`. Type 'help;' or '?' for help.
[default@int4]