Make IFile::isReadOnly consistent.

It is supposed to refer to the property of the file on disk (if there is
any).

Task-number: QTCREATORBUG-4998
Change-Id: Iaed62c17d124b364aecec4d1f910046bade42d40
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>

src/plugins/vcsbase/vcsbaseeditor.h

@@ -113,7 +113,6 @@ public:
      * by default since it should not  trigger when patches are opened as
      * files. */
     void setForceReadOnly(bool b);
-    bool isForceReadOnly() const;
 
     QString source() const;
     void setSource(const  QString &source);