Remove `decode_key_docid/1`

`decode_key_docid/2 was used only once, hence remove it to make the
API smaller.

Change-Id: I09731fcf1cb38e3991c70b9049fdaf24d3c9141a
Reviewed-on: http://review.couchbase.org/44233
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Harsha H S <hhs.couchbase@gmail.com>