Deprecate accumulate

[angelikatyborska]

Following: exercism/problem-specifications#1918

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[jiegillet]

What exactly happens now? It won't appear on the tree since it doesn't;t practice anything, but it will still be in the full list?

[angelikatyborska]

It will disappear completely from anywhere except for students that already started it, they will see it on the full list. E.g. both "binary" and "hexadecimal" are deprecated. I can find "binary" on my exercise list because I solved it, but not "hexadecimal".