[token-server] Remove legacy use strict

Summary: This is now handled by ts compilation and does not need to appear at the top of the file

Test Plan:
grep -r --exclude-dir={node_modules,dist} 'use strict' .

only output is from legacy js files that will not be converted to ts

./.mocharc.js:'use strict';
./.eslintrc.js:'use strict';

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15623