[kcms/user] Drop placeholder email address in KCM

We don't have placeholders in any other field in this KCM.

Showing a placeholder is good to show a default value, but showing an
example of what an email address is with an incorrect value doesn't add
anything.

kcms/users/package/contents/ui/UserDetailsPage.qml

@@ -159,7 +159,6 @@ SimpleKCM {
                 id: emailTextField
                 focus: true
                 text: user.email
-                placeholderText: i18nc("Example email address", "john.doe@kde.org")
                 Kirigami.FormData.label: i18n("Email address:")
             }