[AIX]test failure fix using gtest-1.15.0 for AIX (#21497)

### Description
Local CI setup for AIX reported tests failure after the gtest 1.15.0
upgrade.

### Motivation and Context
Below tests failure is observed after gtest upgrade.

The following tests FAILED:
	  1 - onnxruntime_test_all (ILLEGAL)
	  7 - onnxruntime_logging_apis_test (Subprocess aborted)

To fix this, I am enabling pthread support under gtest. This was
disabled with previous version of gtest for some reason.
Now by enabling this, above tests are getting passed with gtest 1.15.0.

cmake/external/onnxruntime_external_deps.cmake

@@ -46,9 +46,6 @@ if (onnxruntime_BUILD_UNIT_TESTS)
   if (CMAKE_SYSTEM_NAME STREQUAL "Emscripten")
     set(gtest_disable_pthreads ON)
   endif()
-  if (${CMAKE_SYSTEM_NAME} MATCHES "AIX")
-    set(gtest_disable_pthreads ON CACHE BOOL "gtest_disable_pthreads" FORCE)
-  endif()
   set(INSTALL_GTEST OFF CACHE BOOL "" FORCE)
   if (IOS OR ANDROID)
     # on mobile platforms the absl flags class dumps the flag names (assumably for binary size), which breaks passing