Remove legacy (old API) mysql_password function

There's been a properly namespaced modern mysql::password function for
some time. See https://github.com/puppetlabs/puppetlabs-mysql/pull/1044

When the modern API non-namespaced shim was removed in a68004c35ba09fe508020b65a261529437f3a09e
the puppet/parser/functions version still existed so users who hadn't
migrated to mysql::password would have reverted to this legacy API version.

This commit finally removes the legacy version, but puts back the shim
for now (to not create a breaking change just yet).

Both namespaced and non-namespaced shim are tested.