Remove stray (void)0.

Probably a remnant of ifdef soup somewhere.

Change-Id: I472f236a2db54a97490b22b0bbcc1701a2dba3b3
Reviewed-on: https://boringssl-review.googlesource.com/5623
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/bn/exponentiation.c b/crypto/bn/exponentiation.c
index 5ae5b78..a829810 100644
--- a/crypto/bn/exponentiation.c
+++ b/crypto/bn/exponentiation.c
@@ -925,7 +925,6 @@
     }
   }
 #endif
-  (void)0;
 
   /* Allocate a buffer large enough to hold all of the pre-computed
    * powers of am, am itself and tmp.