[Cashtab] Remove dirty param from formData

Summary: In cashtab/components/Send/Send.js, the form data object has a parameter 'dirty' which is no longer being used and should be removed. Related to task T2082.

Test Plan:
cd web/cashtab && npm start
navigate to send tab
send XEC and ensure transaction is successful
send XEC with message and ensure transaction is successful
send XEC with signature and ensure transaction is successful

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10691