Remove trailing whitespaces in various files

Summary:
As per title. This is a required step before introducing a linter that
automates it.

Test Plan:
Read the files and check there is no behavior change.

ninja
make

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5648