Remove Singleton metaclass from NumberSymbol

Subclasses of NumberSymbol should manually set Singleton as their metaclass if
they want to be singletonized.

See issue 3643.
Explicitly make HBar a singleton
Merge pull request #1791 from asmeurer/NumberSymbol-Singleton

Remove Singleton metaclass from NumberSymbol

Subclasses of NumberSymbol should manually set Singleton
as their metaclass if they want to be singletonized.

See issue 3643.