variants: remove the 'built' assertions

variants: remove the 'built' assertions

When using %define parse.assert, the variants come with additional variables
that are useful for development purposes. One is a Boolean indicating if the
variant is built (to make sure we don't read a non-built variant), and the
other is a string describing the stored type. There is no need to have both of
these, the string is enough.

* data/variant.hh (built): Remove.