ANDROID: gki_defconfig: disable FUNCTION_TRACER

Remove CONFIG_FUNCTION_TRACER as it's not needed. Note that
CONFIG_FTRACE is still enabled by default.

Change-Id: Iaf1452937c72325871c1a49b85ba37b4bf136d2e
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>