gnu: elixir: Don't fix Git executable location during Elixir build.

The removed code below affected two locations where Elixir uses Git:

* At build-time, Git is used to see if the build is inside a Git repo and if so, git info
  is added to the build information that Elixir returns with `System.build_info()`. The code
  uses PATH + `git` so the Git version from the inputs is used which is just fine.
* At run-time, Git is used by Mix if, and only if, there are any Git-based dependencies. This
  is not the normal case, and in that sense Git is an optional dependency of Elixir - most
  projects will work just fine without Git being present. Here, too, PATH + `git` is used
  so if there is a need on a user's system to use Git-based dependencies, the user can just
  add the package to the profile and things will just work.

I feel that leaving this alone is cleaner than trying to fix the dependency (if we want to
keep it, it needs to be in propagated-inputs in order to keep Git around; I stumbled upon
this building a project with Git dependencies after doing a `guix gc`. However, given that
using Git dependencies is the exception and I think that minimizing dependencies is nicer
I opted to just remove the substition completely).

* gnu/packages/elixir.scm (elixir)[arguments]: In 'replace-paths' phase,
remove substitutions of "git".

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

gnu/packages/elixir.scm

@@ -60,11 +60,9 @@ (define-public elixir
               (for-each make-file-writable (find-files "."))))
           (add-after 'make-git-checkout-writable 'replace-paths
             (lambda* (#:key inputs #:allow-other-keys)
-              (substitute* '("lib/elixir/lib/system.ex"
-                             "lib/mix/lib/mix/scm/git.ex")
-                (("(cmd\\(['\"])git" _ prefix)
-                 (string-append prefix
-                                (search-input-file inputs "/bin/git"))))
+              ;; Note: references end up obfuscated in binary BEAM files where
+              ;; they may be invisible to the GC and graft code:
+              ;; <https://issues.guix.gnu.org/54304#11>.
               (substitute* '("lib/mix/lib/mix/release.ex"
                              "lib/mix/lib/mix/tasks/release.init.ex")
                 (("#!/bin/sh")