selftests/ftrace: Remove unneeded per-test init/cleanup ftrace

Since ftracetest framework calls initialize_ftrace() right before
each test and after all tests, we don't need to init/cleanup
ftrace for each test case.
Just remove such unneeded init/cleanup code because it can
increase logfile size.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>