; Make erc--send-input-lines a normal function again

; Make erc--send-input-lines a normal function again

* lisp/erc/erc.el (erc--send-input-lines): Revert portion of
174b3dd9bd78c662ce9fff78404dcfa02259d21b "Make nested input handling
more robust in ERC" that converted this from a function to a method.
Instead, defer change until it's needed, likely for bug#49860.  Also,
don't inadvertently allow overloading of `insertp' because user code
can legitimately set that to a function, which we then blindly call.
Instead, hard-code it to the only expected alternate display function.

(cherry picked from commit 74f022b2797567ab04405af37b877d94cc4fdca2)