[Nix docs] Remove duplicated Xcode settings section

Summary:
This section exists in the setup section here:
https://github.com/CommE2E/comm/blob/master/docs/nix_mobile_setup.md#xcode-settings

It seems that a paragraph as accidently included when writing the workflow
documentation.

Mentioned in https://linear.app/comm/issue/ENG-2602#comment-859d7f04

Part of https://linear.app/comm/issue/ENG-2699

Test Plan: Rendered docs: https://github.com/CommE2E/comm/tree/a608ec302c36da42583947fb98d647d2d2ee7db1/docs

Reviewers: ashoat, varun, inka

Reviewed By: ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6235