cocoa_common: drop dead ui_window_cocoa_t stack declaration

-[CocoaView init] at cocoa_common.m:254 had an #if defined(HAVE_COCOA)
block that declared a stack-local ui_window_cocoa_t, assigned its
.data field to self, then let the variable go out of scope without
passing, storing, or otherwise referencing it.  Pure dead code.

Confirmed unused: ui_window_cocoa_t is the payload type for the
ui_window_cocoa_* driver vtable in ui_cocoa.m, but that vtable's
init function returns NULL, so the other entry points never see
a valid ui_window_cocoa_t anyway - unrelated pre-existing plumbing
issue with the ui_window driver system, not something this commit
addresses.

Removing the block also lets the two surrounding #if defined(OSX)
blocks merge into one, eliminating one preprocessor conditional.
Zero runtime change; the generated code for -[CocoaView init] is
identical modulo the unused stack slot.

Caught during a HAVE_COCOA_METAL vs HAVE_COCOA convergence sweep
(following 492b9c4) - one of the few remaining HAVE_COCOA-only
branches turned out to be dead code rather than a real platform
divergence.

Author: LibretroAdmin

ui/drivers/cocoa/cocoa_common.m

@@ -249,17 +249,10 @@ - (id)init
    [self setAutoresizingMask:NSViewWidthSizable | NSViewHeightSizable];
    NSArray *array = [NSArray arrayWithObjects:NSColorPboardType, NSFilenamesPboardType, nil];
    [self registerForDraggedTypes:array];
-#endif
-
-#if defined(HAVE_COCOA)
-   ui_window_cocoa_t cocoa_view;
-   cocoa_view.data = (CocoaView*)self;
-#endif
 
-#if defined(OSX)
-    video_driver_display_type_set(RARCH_DISPLAY_OSX);
-    video_driver_display_set(0);
-    video_driver_display_userdata_set((uintptr_t)self);
+   video_driver_display_type_set(RARCH_DISPLAY_OSX);
+   video_driver_display_set(0);
+   video_driver_display_userdata_set((uintptr_t)self);
 #endif
 
 #if TARGET_OS_TV