Merge #15565: doc: remove release note fragments

6e1aaff doc: remove release note fragments (fanquake)

Pull request description:

  Removes all release note fragments from prior to the 0.18.0 branch off.

  All of these fragments have been merged into the WIP release-notes on the [dev wiki](https://github.com/bitcoin-core/bitcoin-devwiki/wiki/0.18.0-Release-Notes-Draft).

Tree-SHA512: ad991134bdb04c556bf9abf8211c253a65866e20e11ed111b5672270c138c5427608a56df6b155af3f0fd5bd7e8632b322d90dffc51b5d9bf742b85cf63c0c87

Author: laanwj