fix: remove assert_not_nullptr import not available in cairo-lang 0.13.5

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: naor-starkware

vm/src/tests/cairo_test_suite/test_math/math_test.cairo

@@ -2,7 +2,6 @@
 
 from starkware.cairo.common.math import (
     assert_not_zero,
-    assert_not_nullptr,
     assert_not_equal,
     assert_nn,
     assert_le,