samples: remove CONFIG_SERIAL from blinky

As noted in the issue #20666 discussion, though the console is not used
by this sample app, it can't really be "disabled" using this Kconfig
option, so remove the CONFIG_SERIAL=n line.

Fixes: #20666

Signed-off-by: David B. Kinder <[email protected]>

Author: dbkinder

samples/basic/blinky/prj.conf

@@ -1,2 +1 @@
 CONFIG_GPIO=y
-CONFIG_SERIAL=n