fix(ci): move test --cover out of mix ci alias

mix ci previously included 'test --cover' as its last step,
which caused an Elixir 1.20 warning about running in the
wrong environment. Tests are already standalone steps in the
GitHub Actions CI workflow (both Linux and macOS runners).

Changes:
  * Removed 'test --cover' from the ci alias
  * Removed preferred_envs from def project (no longer needed
    since ci no longer chains into test)
  * Removed empty def cli block

mix ci now runs 7 steps: compile, format, credo, deps check,
audit, xref, dialyzer, ex_dna. Tests are run separately with
MIX_ENV=test.

Author: gilbertwong96

mix.exs

@@ -155,7 +155,6 @@ defmodule AdoCli.MixProject do
         "deps.audit",
         "xref graph --label compile-connected --fail-above 0",
         "ci.dialyzer",
-        "test --cover",
         "ex_dna"
       ],
       quality: [
@@ -171,10 +170,6 @@ defmodule AdoCli.MixProject do
     ]
   end
 
-  def cli do
-    [preferred_envs: [test: :test, "test --cover": :test, "test.cover": :test, "coveralls.json": :test]]
-  end
-
   defp escript_config do
     [
       main_module: AdoCli.CLI,