[LINTER] Remove trailing whitespaces

Summary:
In the spirit of the lint-whitespace linter from core, this linter
removes the trailing whitespaces.

The equivalent phpcs rule is no longer needed and is removed.

Test Plan:

arc lint --everything

Introduce trailing whitespaces anywhere the linter is looking (see
.arclint for the exclusions).

arc lint

Check the linter proposes a replacement.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5649