databases/mysql56-{client, server}: Update to 5.7.21
This update fixes bugs like CVE-2018-2696, CVE-2018-2562, CVE-2018-2640,
CVE-2018-2668, CVE-2017-3737 (and more) in MySQL protocol by upstream.
Delete local patches (CMake plugin macros) that are merged by upstream.
PR: 225195
Sponsored by: Netzkommune GmbH