Remove WiredTiger raw compression support

Remove WiredTiger raw compression support:

1. There is no longer any customer use of raw compression,
2. As of the 3.6 release raw compression cannot be used with the lookaside table, making it useless for many workloads (see WT-3659),
3. The raw compression code is relatively complex and brittle.

The changes in WT-3968 (to make compression more adaptive to run-time compression results), partly overlaps with raw compression – raw compression maximizes compression results which is less important if standard compression adapts to do better compression. However, raw compression also allowed targeting fixed I/O sizes, which is potentially useful functionality we'd be getting rid of.