databases/mysql56-server: Adapt ${name}_limits to the new mechanism
Regarding to the changes to rc.subr(8) it is no longer necessary to
define a resource limiting logic in service scripts.
limits(1) is now run by default and its configuration is handled
via the "${name}_limits" variable.
This however causes collision of variable names and also is not
compatible with the old mechanism.
This fixes the rc-script of mysql56 for the bases with both
old and new mechanism.

PR: 227229
Submitted by: 0mp
Reported by: 0mp
Sponsored by: Netzkommune GmbH