ui: remove break after g_assert_not_reached()

Use of assert(false) can trip spurious control flow warnings from
some versions of GCC (i.e. using -fsanitize=thread with gcc-12):

  error: control reaches end of non-void function [-Werror=return-type]
        default:
            g_assert_not_reached();
            break;
          | ^^^^^

Solve that by removing the unreachable 'break' statement, unifying
the code base on g_assert_not_reached() instead.

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240910221606.1817478-37-pierrick.bouvier@linaro.org>
[PMD: Add description suggested by Eric Blake]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
diff --git a/ui/qemu-pixman.c b/ui/qemu-pixman.c
index 5ca55dd..6cada8b 100644
--- a/ui/qemu-pixman.c
+++ b/ui/qemu-pixman.c
@@ -49,7 +49,6 @@
         break;
     default:
         g_assert_not_reached();
-        break;
     }
 
     pf.amax = (1 << pf.abits) - 1;