Hide _S_n_primes from user code

[François Dumont]

Hello

    Here is a rather trivial patch, just code cleanup. Since we export 
_Prime_rehash_policy we do not need to expose the _S_n_primes anymore.

    * include/bits/hashtable_policy.h (_Prime_rehash_policy::_S_n_primes):
    Delete.
    * src/c++11/hashtable_c++0x.cc (_Prime_rehash_policy::_M_next_bkt):
    Remove usage of latter and compute size of the prime numbers array
    locally.

Tested under Linux x86_64.

Ok to commit ?

François