Revert "kernel: Deprecate CONFIG_MULTITHREADING"

This reverts commit 28cb9dab64.

kernel/CMakeLists.txt

@@ -65,10 +65,6 @@ if(${CONFIG_KERNEL_MEM_POOL})
   target_sources(kernel PRIVATE mempool.c)
 endif()
 
-if(NOT CONFIG_MULTITHREADING)
-  message(WARNING "Single threaded mode (CONFIG_MULTITHREADING=n) is deprecated")
-endif()
-
 # The last 2 files inside the target_sources_ifdef should be
 # userspace_handler.c and userspace.c. If not the linker would complain.
 # This order has to be maintained. Any new file should be placed

kernel/Kconfig

@@ -10,11 +10,9 @@ module-str = kernel
 source "subsys/logging/Kconfig.template.log_config"
 
 config MULTITHREADING
-	bool "Multi-threading (DEPRECATED)"
+	bool "Multi-threading"
 	default y
 	help
-	  Disabling this option is DEPRECATED.
-
 	  If disabled, only the main thread is available, so a main() function
 	  must be provided. Interrupts are available. Kernel objects will most
 	  probably not behave as expected, especially with regards to pending,
@@ -25,11 +23,6 @@ config MULTITHREADING
 	  set to 'n'; disable only when you REALLY know what you are
 	  doing.
 
-if !MULTITHREADING
-comment "*** WARNING ***"
-comment "Single threaded mode (MULTITHREADING option disabled) is deprecated"
-endif
-
 config NUM_COOP_PRIORITIES
 	int "Number of coop priorities" if MULTITHREADING
 	default 1 if !MULTITHREADING