Drop wgShortUrlPrefix

[hexmode]

I cannot find any references to it except in SESP in code search.

@kghbln ... I see you and @mwjames talking about it in #88 but otherwise, nada.

[hexmode]

At worst, I think we can change it to sespShortUrlPrefix

[kghbln]

wgShortUrlPrefix was not changed since it is a parameter not defined by SESP (See #88 (comment)) I speculate that extension ShortUrl dropped this parameter in the meantime, meaning sespgShortUrlPrefix could be defined here to reinstate integration with that extension.