Revert D22208758: [pytorch][PR] Report error when ATEN_THEADING is OMP and USE_OPENMP is turned off.

Test Plan: revert-hammer

Differential Revision:
D22208758 (pytorch@3ed96e4)

Original commit changeset: 0866c9bb9b3b

fbshipit-source-id: 9e2b469469e274292b2559c02aa0256425fd355e

Author: pbelevich

caffe2/CMakeLists.txt

@@ -22,15 +22,12 @@ set(AT_PARALLEL_NATIVE_TBB 0)
 
 message(STATUS "Using ATen parallel backend: ${ATEN_THREADING}")
 if("${ATEN_THREADING}" STREQUAL "OMP")
-  if(NOT USE_OPENMP)
-    message(FATAL_ERROR "ATen is using OpenMP backend but USE_OPENMP is off. Please either change ATEN_THREADING or turn on USE_OPENMP.")
-  endif()
   set(AT_PARALLEL_OPENMP 1)
 elseif("${ATEN_THREADING}" STREQUAL "NATIVE")
   set(AT_PARALLEL_NATIVE 1)
 elseif("${ATEN_THREADING}" STREQUAL "TBB")
   if(NOT USE_TBB)
-    message(FATAL_ERROR "ATen is using TBB backend but USE_TBB is off. Please either change ATEN_THREADING or turn on USE_TBB.")
+    message(FATAL_ERROR "Using TBB backend but USE_TBB is off")
   endif()
   set(AT_PARALLEL_NATIVE_TBB 1)
 else()