LibVideo: Remove MV class's copy assignment overload

This was unnecessary, as the implicit one works correctly.

Userland/Libraries/LibVideo/VP9/MV.cpp

@@ -14,15 +14,6 @@ MV::MV(u32 row, u32 col)
 {
 }
 
-MV& MV::operator=(MV const& other)
-{
-    if (this == &other)
-        return *this;
-    m_row = other.row();
-    m_col = other.col();
-    return *this;
-}
-
 MV& MV::operator=(i32 value)
 {
     m_row = value;

Userland/Libraries/LibVideo/VP9/MV.h

@@ -20,7 +20,6 @@ public:
     u32 col() const { return m_col; }
     void set_col(u32 col) { m_col = col; }
 
-    MV& operator=(MV const& other);
     MV& operator=(i32 value);
     MV operator+(MV const& other) const;