Drop setting OMP_NUM_THREADS

It was workaround before tarantool/tarantool#2431
was fixed (see comment [1] in the issue). Now we are going to drop sort
using OpenMP so it is time to drop this workaround altogether.

[1] tarantool/tarantool#2431 (comment)

Part of tarantool/tarantool#3389

Author: nshy

test-run.py

@@ -323,10 +323,6 @@ def open_as_utf8(*args, **kwargs):
 
         __builtins__.open = open_as_utf8
 
-    # don't sure why, but it values 1 or 2 gives 1.5x speedup for parallel
-    # test-run (and almost doesn't affect consistent test-run)
-    os.environ['OMP_NUM_THREADS'] = '2'
-
     status = 0
 
     if Options().args.show_tags: