Remove unused method

Summary:
This isn't used and if it was would give the wrong value.
Actual correct size can be determined via Screens object.

Test Plan:
Grepped it's not overriding anything
Compiles

Reviewers: #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3385

plugins/platforms/drm/drm_backend.cpp

@@ -664,14 +664,6 @@ void DrmBackend::moveCursor()
     }
 }
 
-QSize DrmBackend::size() const
-{
-    if (m_outputs.isEmpty()) {
-        return QSize();
-    }
-    return m_outputs.first()->size();
-}
-
 Screens *DrmBackend::createScreens(QObject *parent)
 {
     return new DrmScreens(this, parent);

plugins/platforms/drm/drm_backend.h

@@ -76,7 +76,6 @@ public:
     DrmBuffer *createBuffer(gbm_surface *surface);
     void present(DrmBuffer *buffer, DrmOutput *output);
 
-    QSize size() const;
     int fd() const {
         return m_fd;
     }