powerpc: Remove inaccessible CMDLINE default

Since commit cbe46bd4 ("powerpc: remove CONFIG_CMDLINE #ifdef mess")
CONFIG_CMDLINE has always had a value regardless of CONFIG_CMDLINE_BOOL.

For example:

 $ make ARCH=powerpc defconfig
 $ cat .config
 # CONFIG_CMDLINE_BOOL is not set
 CONFIG_CMDLINE=""

When enabling CONFIG_CMDLINE_BOOL this value is kept making the 'default
"..." if CONFIG_CMDLINE_BOOL' ineffective.

 $ ./scripts/config --enable CONFIG_CMDLINE_BOOL
 $ cat .config
 CONFIG_CMDLINE_BOOL=y
 CONFIG_CMDLINE=""

Remove CONFIG_CMDLINE_BOOL and the inaccessible default.
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200611224220.25066-2-chris.packham@alliedtelesis.co.nz

arch/powerpc/Kconfig

@@ -859,12 +859,8 @@ config PPC_DENORMALISATION
 	  Add support for handling denormalisation of single precision
 	  values.  Useful for bare metal only.  If unsure say Y here.
 
-config CMDLINE_BOOL
-	bool "Default bootloader kernel arguments"
-
 config CMDLINE
-	string "Initial kernel command string" if CMDLINE_BOOL
-	default "console=ttyS0,9600 console=tty0 root=/dev/sda2" if CMDLINE_BOOL
+	string "Initial kernel command string"
 	default ""
 	help
 	  On some platforms, there is currently no way for the boot loader to