Remove unsed header from singleton-inl.h

Summary:
This was recently added by @andrii to provide stack traces,
but it no longer needed since he now produces them from a hook in
folly/experimental/SingletonStackTrace.cpp

Test Plan: Continues to build without the include.

Reviewed By: andrii@fb.com

Subscribers: folly-diffs@, yfeldblum, andrii

FB internal diff: D1842654

Signature: t1:1842654:1423696133:cfa91851f28696082f790eb13c3b0b015c813590

folly/experimental/Singleton-inl.h

@@ -13,7 +13,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-#include <folly/experimental/symbolizer/Symbolizer.h>
 
 namespace folly {