Revert "cache the serialization of a Key"

Revert "cache the serialization of a Key"

This reverts commit 4536c93bb2ecf114ab711beac33fa358facd6985.

That broke Read/Show of a Key, and unfortunately Key is read in at least
one place; the GitAnnexDistribution data type.

It would be worth bringing this optimisation back, but it would need
either a custom Read/Show instance that preserves back-compat, or
wrapping Key in a data type that contains the serialization, or changing
how GitAnnexDistribution is serialized.

Also, the Eq instance would need to compare keys with and without a
cached seralization the same.