Remove "use strict" directive from ES Modules

Summary:
ES Modules implicitly enable strict mode. Adding the "use strict" directive is, therefore, not required.

This diff removes all "use strict" directives from ES modules.

Changelog:

[Internal]

Reviewed By: motiz88

Differential Revision: D26172715

fbshipit-source-id: 57957bcbb672c4c3e62b1db633cf425c1c9d6430