x86: MMIOTRACE should not default to on

Even the help-text makes it clear that normal people shouldn't enable
it.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/Kconfig.debug

@@ -179,7 +179,6 @@ config MMIOTRACE
 	depends on DEBUG_KERNEL && PCI
 	select TRACING
 	select MMIOTRACE_HOOKS
-	default y
 	help
 	  Mmiotrace traces Memory Mapped I/O access and is meant for
 	  debugging and reverse engineering. It is called from the ioremap