remove xcode dependency

[thedavidmeister]

adding clang in holochain/holonix#57 seemed to make everything work for hc without xcode

i have no idea if there is anything else that would need to be done to support development more generally without xcode - i.e. in the standard nix-shell environment

nix shell is much more general purpose than what was done in 57 (that only targets what hc does internally) but it would be very cool if we could remove the xcode dependency for mac users as it's rather heavy to have both xcode and nix shell, e.g. if you're running a mac air

CC: @dhtnetwork

[thedavidmeister]

mentioned here https://forum.holochain.org/t/im-spinning-up-some-docs-for-holonix-feedback-welcome/451/31?u=thedavidmeister

CC: @jmday