fix: Remove unused import of @blockly/field-colour

This import should have been removed in PR #129 when the code
that uses it was rewritten.

Fixes #133.

Author: cpcallen

src/line_cursor.ts

@@ -13,7 +13,6 @@
  * @author [email protected] (Abby Schmiedt)
  */
 
-import {FieldColour} from '@blockly/field-colour';
 import * as Blockly from 'blockly/core';
 import {ASTNode, Marker} from 'blockly/core';