[Nix] Remove flakeConfig for cache

Summary:
Now that install the binary caches through the
./script/install_nix.sh script, remove flakeConfig which doesn't appear
to be working as intended for substituters

Cleanup item for https://linear.app/comm/issue/ENG-2126

Depends on https://phab.comm.dev/D5561

Test Plan:

nix develop

# no longer emits "Using saved setting for ..."

Reviewers: atul, varun

Reviewed By: atul

Subscribers: ashoat, tomek, abosh

Differential Revision: https://phab.comm.dev/D5562