Move PHPCS config file where it is used

Summary: As per title.

Test Plan:

arc lint--everything

PHPCS still complains like before.

Reviewers: #bitcoin_abc, jasonbcox, Fabien

Reviewed By: #bitcoin_abc, jasonbcox, Fabien

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3710