compound ket fn: remove-prefix

remove-prefix:
    description:
        remove-prefix["str"] ket
        if ket has prefix str then remove it

    examples:
        remove-prefix["not "] |not sitting at the beach>
            |sitting at the beach>

        remove-prefix["word: "] |word: table>
            |table>

        remove-prefix["food: "] |word: chair>
            |word: chair>

    see also:
        remove-suffix, has-suffix, has-prefix