[Ref] Simplify is_email_receipt in sendMail

[Ref] Simplify is_email_receipt in sendMail

This function has evolved through refactoring so there was a time when this code needed to decide whether to
send emails out or not. However, the function is now called more deliberately -ie
1) from completeTransaction - where it is only called if the decision to send an email is made
2) from the search action send Email receipts - where a user has made the decision to send (or pdf)
receipts and unless it's being called with returnMessageText = TRUE (to construct a pdf) then
send makes sense

Ergo we should send unless returnMessageText = TRUE