WikiPage: Remove premature Content::isValid check

Content classes that implement the isValid method are causing
an "Internal error" to appear when saving from EditPage because
WikiPage throws an exception that isn't handled anywhere.

Turns out that ContentHandler already has infrastructure for
this in place in Content::prepareSave (which returns a Status
object with a relevant message informing the user), and WikiPage
is actually calling this already right after this.

Content::prepareSave defaults to using Content::isValid, so this
code is simply obsolete and premature as it calls an internal
method directly without handling it accordingly.

Change-Id: I69f18d9e101c2261b0ff7bd1b0459ea7fdd7ae9f