UI: Fix Qt AutoUic warning

Currently, Qt's User Interface Compiler (UIC) emits a warning about the
tabstop assignment for horizontalLayout_sdrPaperWhite, which is a
QLayout rather than a QWidget. Remove the tabstop assignment to fix the
warning.

UI/forms/OBSBasicSettings.ui

@@ -7664,7 +7664,6 @@
   <tabstop>colorSpace</tabstop>
   <tabstop>colorRange</tabstop>
   <tabstop>sdrWhiteLevel</tabstop>
-  <tabstop>horizontalLayout_sdrPaperWhite</tabstop>
   <tabstop>hdrNominalPeakLevel</tabstop>
   <tabstop>disableOSXVSync</tabstop>
   <tabstop>resetOSXVSync</tabstop>