[RF] Remove `NormFactor::GetConst()` and `SetConst()` member functions

As printed out by the HistFactory in a warning message for a long time
already, setting the `Const` attribute to the `<NormFactor>` tag is
deprecated and it will be ignored. Instead, add
`<ParamSetting Const="True"> myparam </ParamSetting>`
to your top-level XML's `<Measurement>` entry.

This deprecation implied that the constant parameter flag in the
`RooStats:HistFactory::NormFactor` class had no effect as well. To avoid
ambiguity in the future, the possibility to set and retrieve this flag with
`NormFactor::SetConst()` and `NormFactor::GetConst()` was removed, and the
aforementioned deprecated warning is not printed anymore.