bug: billing_note duplication each time invoice form is saved

[surfacehazard]

Describe the bug

This PR, adds the ability to edit the encounter billing note. However, since the textarea contains both the patient billing note and the encounter billing note, the encounter billing note will be incorrectly appended to and will continue to be duplicated each time the invoice form is saved, due to this line.

[stephenwaite]

thanks @surfacehazard !