tests: Disable test for freed disk space that fails on Btrfs.

This is a follow-up to commit 40e89f5be6.

* tests/store.scm ("dead path can be explicitly collected"): Don't check
that (> freed 0).
* gnu/packages/package-management.scm (guix)[arguments]<#:phases>: Remove
workaround for the same problem.

gnu/packages/package-management.scm

@@ -182,12 +182,6 @@
                           (substitute* "tests/guix-environment-container.sh"
                             (("guix environment --version")
                              "exit 77\n")))
-                        ;; XXX: On some file systems (notably Btrfs), the test
-                        ;; for freed disk space after GC may fail.  See
-                        ;; <https://bugs.gnu.org/29363>.
-                        (substitute* "tests/store.scm"
-                          (("(\\(> freed 0\\))" all)
-                           (string-append ";" all)))
                         #t))
                     (add-before 'check 'set-SHELL
                       (lambda _

tests/store.scm

@@ -180,7 +180,9 @@
                               (random-text) '())))
     (let-values (((paths freed) (delete-paths %store (list p))))
       (and (equal? paths (list p))
-           (> freed 0)
+           ;; XXX: On some file systems (notably Btrfs), freed
+           ;; may return 0.  See <https://bugs.gnu.org/29363>.
+           ;;(> freed 0)
            (not (file-exists? p))))))
 
 (test-assert "add-text-to-store vs. delete-paths"