Delete Unused Classes

The classes
`org.javamoney.moneta.convert.DefaultMonetaryConversionsSingletonSpi`
and `org.javamoney.moneta.convert.TestMonetaryConversionSingletonSpi`
are not referenced anywhere and are not tests either. They can be
deleted and the number of tests stays the same and no tests fail.
Merge pull request #49 from marschall/delete-classes

Delete Unused Classes