Fix "Text is read-only" on backspacing initial Calc input

Fix "Text is read-only" on backspacing initial Calc input

Immediately after `calc-mode' opens the minibuffer for input
(because you typed a digit, "e", etc), pressing backspace
should clear the minibuffer and return you to the *Calculator*
buffer.  Instead, it leaves the minibuffer as-is and prints the
message "Text is read-only"; this is because the function used,
`erase-buffer', tries to erase the read-only minibuffer prompt.
Using `delete-minibuffer-contents' fixes this, since it doesn't
attempt to delete the prompt.
* lisp/calc/calc.el (calcDigit-backspace): Use
`delete-minibuffer-contents' instead of `erase-buffer'.  (Bug#67395)

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