Don't use -dbcache for BDB anymore

-dbcache was originally used to set the maximum buffer size in the
BDB environment, and was later changed to set the chainstate cache
and leveldb caches. No need to use it for BDB now that only the
wallet remains there.

This should reduce memory allocation (but not necessarily memory
usage) a bit.
Merge pull request #2422 from sipa/nowalcache

Don't use -dbcache for BDB anymore