Remove unnecessary fence in deque_pop.

Benchmark results (with gcc-4.8.0 -O2):

[jackyang@les:~/workspace/fiber]$ ./fib 48 42
fib(42)=267914296 (0.936875 seconds)
[jackyang@les:~/workspace/fiber]$ ./fib 32 42
fib(42)=267914296 (1.344375 seconds)
[jackyang@les:~/workspace/fiber]$ ./fib 16 42
fib(42)=267914296 (2.724375 seconds)
[jackyang@les:~/workspace/fiber]$ ./fib 8 42
fib(42)=267914296 (5.345000 seconds)
[jackyang@les:~/workspace/fiber]$ ./fib 4 42
fib(42)=267914296 (10.550000 seconds)
[jackyang@les:~/workspace/fiber]$ ./fib 2 42
fib(42)=267914296 (20.970000 seconds)
[jackyang@les:~/workspace/fiber]$ ./fib 1 42
fib(42)=267914296 (41.830000 seconds)
Merge branch 'master' of github.com:chaoran/fiber