Revert "SpecialNewLexeme: use const for name"
Using SpecialNewLexeme::PAGE_NAME requires the class to be loaded
unconditionally, which in turn requires its superclass,
SpecialNewEntity, to be available. However, that class is not available
on wikis where WikibaseRepo is not enabled, resulting in an error.
This reverts commit a68c5a8dd88fd47b7c588f810fabaa5b5877b11e.