Simplify default text props for ERC input

Simplify default text props for ERC input

* lisp/erc/erc.el (erc--msg-props): Reformat doc string to improve
readability.
(erc-send-current-line): Don't bind `erc--msg-prop-overrides' because
doing so affects all scoped calls to `erc-display-message' made by
slash commands, etc. and makes handling overly complex and error
prone.
(erc-display-msg): Set `erc-msg' property to `msg' instead of `self'
because the only legitimate use of this function is for inserting
"echoed" prompt input.  (Bug#60936 and Bug#67031)