[users] Misc cleanups

- unused includes
- avoid "my--pc" .. the dash is inserted by makeHostnameSuggestion()

src/modules/users/Config.cpp

@@ -221,7 +221,7 @@ guessProductName()
         }
         if ( dmiProduct.isEmpty() )
         {
-            dmiProduct = QStringLiteral( "-pc" );
+            dmiProduct = QStringLiteral( "pc" );
         }
         tried = true;
     }

src/modules/users/UsersPage.cpp

@@ -43,8 +43,6 @@
 #include <QFile>
 #include <QLabel>
 #include <QLineEdit>
-#include <QRegExp>
-#include <QRegExpValidator>
 
 /** @brief How bad is the error for labelError() ? */
 enum class Badness