bug#67395: Fix "Text is read-only" on backspacing initial calc input

[Eli Zaretskii]

> From: George Kuzler <gkuzler@gmail.com>
> Date: Wed, 22 Nov 2023 21:02:31 -0500
> 
> This is a one-line fix for a minor issue in calc-mode. When your last
> command has caused the minibuffer to open for input - for example,
> you've typed a digit - and you immediately press backspace, it should
> clear the minibuffer and return you to *Calculator*. Instead, the
> minibuffer is unchanged, and emacs prints the message "Text is
> read-only". Note that you can then backspace again to delete the
> minibuffer contents as normal; this bug only manifests if you try to
> backspace right after the command that puts you in the minibuffer.
> 
> The issue is that `calcDigit-backspace' tries to clear the minibuffer with
> `erase-buffer', which also tries to delete the prompt, which is
> read-only. `delete-minibuffer-contents' would have the desired effect
> since it doesn't try to delete the prompt. This change just replaces the
> former function with the latter.

Thanks, I installed this on the emacs-29 branch, and I'm closing the
bug.