Drop useless and wrong POST parameter

Fixes 6b56cd71dfb112290c88027d1b526f5af9268df5
Revert "Allow modifying zero-quantity orders"

This reverts commits 9b0551835dea4501f0407172cd5ead6b07e1a8cf and
9ab4e797e904e823e11aaf0182b3f2afe9a7b78e.