powerpc/configs: Remove unnecessary ftrace symbols

In commit 539df7fc ("powerpc/configs: Enable function trace by
default") we added:

  CONFIG_FTRACE=y
  CONFIG_FUNCTION_TRACER=y
  CONFIG_FUNCTION_GRAPH_TRACER=y

To ppc64_defconfig, powernv_defconfig and pseries_defconfig.

But only CONFIG_FUNCTION_TRACER=y is required, CONFIG_FTRACE is
default y if DEBUG_KERNEL is enabled, which we have. And then
CONFIG_FUNCTION_GRAPH_TRACER is default y when CONFIG_FUNCTION_TRACER
is enabled.

The extra symbols were already removed from powernv_defconfig in
commit 9a018fb1 ("powerpc/config: powernv_defconfig updates").

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>