pkgs/README.md: Remove some mediocre duplicated sentences

Also mainly covered in the section below

pkgs/README.md

@@ -545,15 +545,6 @@ This is how the pull request looks like in this case: [https://github.com/NixOS/
 
 ## Package tests
 
-- update pkg
-  - `nix-env -iA pkg-attribute-name -f <path to your local nixpkgs folder>`
-- add pkg
-  - Make sure it’s in `pkgs/top-level/all-packages.nix`
-  - `nix-env -iA pkg-attribute-name -f <path to your local nixpkgs folder>`
-- _If you don’t want to install pkg in you profile_.
-  - `nix-build -A pkg-attribute-name <path to your local nixpkgs folder>` and check results in the folder `result`. It will appear in the same directory where you did `nix-build`.
-- If you installed your package with `nix-env`, you can run `nix-env -e pkg-name` where `pkg-name` is as reported by `nix-env -q` to uninstall it from your system.
-
 To run the main types of tests locally:
 
 - Run package-internal tests with `nix-build --attr pkgs.PACKAGE.passthru.tests`