Revert "Parse pre-check and post-check cache control values."

This reverts commit 4af2b333e10e49e19e051e29de6225c2fe7cec92.

This change was probably useless, as per https://github.com/jech/polipo/issues/21
Revert "RFC5861 control values parsing"

This reverts commit 7a8ff3334832e77ee0f4273b6f04d03b858b90d1.

Doesn't do anything useful, see https://github.com/jech/polipo/issues/21