fix: remove unreachable code path causing warning (#718)

Remove the unreachable code path that was causing the following warning
to be shown each time the test suite was run in a project that uses
ash_sql under elixir 1.20.0-rc.3-otp-28

warning: the following clause cannot match because the previous clauses already matched all possible values:

         version ->

     it attempts to match on the result of:

         System.get_env("ASH_VERSION")

     which has the already matched type:

         dynamic(nil or binary())

     where "version" was given the type:

         # type: dynamic(nil or binary())
         # from: mix.exs:114:7
         version

     type warning found at:
     │
 114 │       version ->
     │               ~
     │
     └─ mix.exs:114:15: AshSql.MixProject.ash_version/1

Author: ahey

mix.exs

@@ -222,11 +222,8 @@ defmodule AshPostgres.MixProject do
       "main" ->
         [git: "https://github.com/ash-project/ash.git", override: true]
 
-      version when is_binary(version) ->
-        "~> #{version}"
-
       version ->
-        version
+        "~> #{version}"
     end
   end
 
@@ -241,11 +238,8 @@ defmodule AshPostgres.MixProject do
       "main" ->
         [git: "https://github.com/ash-project/ash_sql.git"]
 
-      version when is_binary(version) ->
-        "~> #{version}"
-
       version ->
-        version
+        "~> #{version}"
     end
   end