[FIX] Wiki: remove possibility of saving the sandbox page (regression)

I removed the possibility of saving the sandbox page (which differentiates it from a normal wiki page) because the sandbox feature which is a wiki page (named "Sandbox"), can just be edited and previewed but can't be saved, and versions will not be recorded

Link to the avantech task: https://avan.tech/item76872 And the related description on dev.tiki.org : https://dev.tiki.org/item8113