As requested by MarcoFalke in https://github.com/bitcoin/bitcoin/pull/16273#issuecomment-521332089:

This PR removes unused includes.

Please note that in contrast to #16273 I'm limiting the scope to the trivial cases of pure removals (i.e. no includes added) to make reviewing easier.

I'm seeking "Concept ACK":s for this obviously non-urgent minor cleanup.

Rationale:
* Avoids unnecessary re-compiles in case of header changes.
* Makes reasoning about code dependencies easier.
* Reduces compile-time memory usage.
* Reduces compilation time.
* Warm fuzzy feeling of being lean :-)

ryanofsky:
  Code review ACK 084e17cebd424b8e8ced674bc810eef4e6ee5d3b. PR only removes include lines and it still compiles. In the worst case someone might have to explicitly add an include later for something now included implicitly. But maybe some effort was taken to avoid this, and it wouldn't be a tragedy anyway.

ninja check-all
ninja