X-Powered-By: remove the setTo option

[EvanHahn]

The X-Powered-By middleware should remove the setTo option in the next version.

• We can dramatically simplify the middleware (at middlewares/x-powered-by/index.ts). We should be able to completely remove XPoweredByOptions and make xPoweredBy take no arguments.
• We should remove this test:

  helmet/test/x-powered-by.test.ts

  Lines 23 to 27 in 936cd27

  	it("allows you to explicitly set the header", async () => {
  	await check(xPoweredBy({ setTo: "steampowered" }), {
  	"x-powered-by": "steampowered",
  	});
  	});

• We should remove the setTo docs from the README.
• We should update the changelog and link to https://github.com/helmetjs/helmet/wiki/How-to-set-a-custom-X%E2%80%93Powered%E2%80%93By-header for people who need help migrating.

The pull request should be made against the 4.x branch, which you can see in #197.

[EvanHahn]

Closed in #226.