config.json: deprecate counter
config.json: deprecate counter

As of https://github.com/exercism/x-common/issues/80 we'd like to keep
the scope of Exercism focused, and counter is the single exercise that
falls under the write-the-test-suite category.

robphoenix/exercism-go,exercism/xgo,leenipper/xgo,robphoenix/exercism-go,exercism/xgo,leenipper/xgo