Removes Noise Reduction settings from Preferences

- Noise Reduction settings removed from
  Preferences > Performance & Quality
- Performance & Quality renamed to Performance, container frame renamed
  to Threads, since it applies to more than just the NR tool.
- Localization keys removed.
- TP_RETINEX_GAINTRANSMISSION_TOOLTIP key revised, closes #3299

rtgui/preferences.h

@@ -162,7 +162,7 @@ class Preferences : public Gtk::Dialog, public ProfileStoreListener
     Gtk::CheckButton* filmStripOverlayedFileNames;
     Gtk::CheckButton* sameThumbSize;
 
-    Gtk::SpinButton*  rgbDenoiseTreadLimitSB;
+    Gtk::SpinButton*  threadsSpinBtn;
     Gtk::SpinButton*  clutCacheSizeSB;
     Gtk::SpinButton*  maxInspectorBuffersSB;
     Gtk::ComboBoxText *thumbnailInspectorMode;