To remove dates from tasks you can use the task input shortcuts (due, +, starts, ends) and type none.
Click the task title (or double click depending on your settings) and start typing the shortcut followed by none.
Bulk removing dates
The same shortcut also works for bulk editing.
Use the shift key to select the first and last task you want to edit. Or use the cmd key to pick individual tasks.
When the bulk editor pops up at the top simply start typing the shortcut.