Remove custom proxy functionality

[gr2m]

Looks like Undici added native proxy support, so we may be able to remove the custom proxy support we added in #102.

create-github-app-token/lib/request.js

Lines 15 to 16 in 3cef845

	// Native support for proxies in Undici is under consideration: https://github.com/nodejs/undici/issues/1650
	// Until then, we need to use a custom fetch function to add proxy support.

Originally posted by @parkerbxyz in #132 (comment)

[parkerbxyz]

This depends on nodejs/node#43187. We may need to wait until that is resolved to finish this.