[c2] Fix flaky test_spatial_bn_multi_batch_grad
Summary: Removed the deadline restriction since the first run can take more than the deadline, wile subsequent runs are shorter.
Reviewed By: ngimel
Differential Revision: D28260077
fbshipit-source-id: 8ed2f5c16bc184bf4fae0a59b662fa1da2d4dd0a
