[S390] Remove kexec experimental flag.

Follow other architectures and remove kexec experimental flag.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/Kconfig

@@ -460,8 +460,7 @@ config S390_HYPFS_FS
 	  information in an s390 hypervisor environment.
 
 config KEXEC
-	bool "kexec system call (EXPERIMENTAL)"
-	depends on EXPERIMENTAL
+	bool "kexec system call"
 	help
 	  kexec is a system call that implements the ability to shutdown your
 	  current kernel, and to start another kernel.  It is like a reboot