Stop always storing blocks from whitelisted peers

Summary:
There is no reason to wish to store blocks on disk always just
because a peer is whitelisted. This appears to be a historical
quirk to avoid breaking things when the accept limits were added.

Partial backport of Core PR 11531 (3d9c70ca0f):
https://github.com/bitcoin/bitcoin/pull/11531/commits/3d9c70ca0fd5d42f31114b689714af1825f0ed30

Test Plan: test_runner.py p2p-acceptblock

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2246