Firestore/core/test/unit/util/CMakeLists.txt: remove set(CMAKE_CXX_EXTENSIONS ON), which was added 7 years ago in #715

Author: dconeybe

Firestore/core/test/unit/util/CMakeLists.txt

@@ -12,8 +12,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-set(CMAKE_CXX_EXTENSIONS ON)
-
 # Required to allow 0 length printf style strings for testing purposes.
 if(NOT MSVC)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-format-zero-length")