zaml: unify on a single definition of to_zaml for Symbol

There were two competitive definitions of the to_zaml function over Symbol,
one in monkey_patches and one in the core ZAML library.

The monkey patch version won in practice, so now they are unified, and the
monkey patch version is adopted to replace the core ZAML version, in the zaml
Ruby file.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>