planner: remove Bühlmann factor cache

The Bühlmann factors were cached in a thread-safe hashmap. It seemed
somewhat dubious that entering a critical section and doing a hash-lookup
would be significantly faster than a simple exp() call.

Indeed, in a very cache friendly test (16 entries, tight loop) calling the
factor() function 32 000 000 times from a different translation units we get:
  - with cache: 604 ms
  - without cache: 266 ms
Therefore, remove the cache. Given that 32 000 000 calls take only 266 ms,
it appears not sensible to try to optimize this function anyway.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

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