Remove special-casing for OpenBSD pthread handling

Previously it was policy to use -pthread, but OpenBSD now recommends -lpthread.
its been libpthread anyway and policy has changed to stop using -pthread.

configure

@@ -764,9 +764,6 @@ if [ "$thread" = "auto" ]; then
                 thread="win32"
             fi
             ;;
-        OPENBSD)
-            cc_check pthread.h -pthread && thread="posix" && libpthread="-pthread"
-            ;;
         *)
             cc_check pthread.h -lpthread && thread="posix" && libpthread="-lpthread"
             ;;