perf/x86/intel/pt: Drop pointless NULL assignment.

Only a few lines below this removed line is this:

  attrs = kzalloc(size, GFP_KERNEL);

and since there is no code path where this could be avoided, the
NULL assignment is a pointless relic of history and can be removed.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200408235216.108980-1-paul.gortmaker@windriver.com