UI: remove UI_BTYPE_SEARCH_MENU_UNLINK

Internal change only,
use UI_BTYPE_SEARCH_MENU with an unlink flag instead.

They are really the same button type, one just happens to have the option to unlink.

source/blender/editors/include/UI_interface.h

@@ -185,6 +185,7 @@ enum {
 	UI_BUT_HAS_SEP_CHAR    = (1 << 27),  /* but->str contains UI_SEP_CHAR, used for key shortcuts */
 	UI_BUT_TIP_FORCE       = (1 << 28),  /* force show tooltips when holding option/alt if U's USER_TOOLTIPS is off */
 	UI_BUT_TEXTEDIT_UPDATE = (1 << 29),  /* when widget is in textedit mode, update value on each char stroke */
+	UI_BUT_SEARCH_UNLINK   = (1 << 30),  /* show unlink for search button */
 };
 
 #define UI_PANEL_WIDTH          340
@@ -279,7 +280,6 @@ typedef enum {
 	UI_BTYPE_WAVEFORM               = (49 << 9),
 	UI_BTYPE_VECTORSCOPE            = (50 << 9),
 	UI_BTYPE_PROGRESS_BAR           = (51 << 9),
-	UI_BTYPE_SEARCH_MENU_UNLINK     = (52 << 9),
 	UI_BTYPE_NODE_SOCKET            = (53 << 9),
 	UI_BTYPE_SEPR                   = (54 << 9),
 	UI_BTYPE_SEPR_LINE              = (55 << 9),