perf ftrace: Remove needless code setting default tracer

perf ftrace: Remove needless code setting default tracer

As a result of commit a3497642c261 ("perf ftrace: Make 'function_graph'
be the default tracer") the ftrace.tracer variable can't be NULL but the
other code setting default tracer remained.

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1485423339-22780-1-git-send-email-treeze.taeung@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>