[dxvk] Simplify discardBuffer access flags

Specifying both read and write access is unnecessary, as the dirty
check will always return true for writes when it would also return
true for reads.

src/dxvk/dxvk_context.cpp

@@ -876,11 +876,7 @@ namespace dxvk {
   
   void DxvkContext::discardBuffer(
     const Rc<DxvkBuffer>&       buffer) {
-    DxvkAccessFlags accessFlags(
-      DxvkAccess::Read,
-      DxvkAccess::Write);
-    
-    if (m_barriers.isBufferDirty(buffer->slice(), accessFlags))
+    if (m_barriers.isBufferDirty(buffer->slice(), DxvkAccess::Write))
       this->invalidateBuffer(buffer, buffer->allocPhysicalSlice());
   }