oprofile, x86: Simplify init/exit functions

Now, that we only call the exit function if init succeeds with commit:

 979048e1

 oprofile: don't call arch exit code from init code on failure

we can simplify the x86 init/exit functions too. Variable using_nmi
becomes obsolete.

Signed-off-by: Robert Richter <robert.richter@amd.com>