Delete khash member mask and inc

because there can be substituted by n_buckets.

and can reduce memory just a little. :ant: 

```
$ uname -a
Linux localhost.localdomain 2.6.32-279.el6.x86_64 #1 SMP Fri Jun 22 12:19:21 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux
$ valgrind --leak-check=full build/host/test/mrbtest
```

before

```
total heap usage: 62,928 allocs, 62,928 frees, 4,322,298,990 bytes allocated
```

after

```
total heap usage: 62,928 allocs, 62,928 frees, 4,322,281,334 bytes allocated
```