Revert several OFHashMap related commits.

This reverts:
 * "Add a per-hashtable seed."

 * "OFMapTable: Reseed on resize."

 * "Add of_random()."

 * "OFMapTable: Rotate hash by a random number of bits"


It turned out that these were slowing down all classes based on
OFHashMap significantly. As there is still no proof that the seeded
one-at-a-time hash is vulnerable to DoS, these changes were overly
paranoid. While overly paranoid is usually ok, it is not if it severely
impacts performance.