Revert "gnu: gnu: Make alsa find its plugins by setting ALSA_PLUGIN_DIR."

This reverts commit 2101cc3fb6.

With commit 1e3861eb, we can do it without ALSA_PLUGIN_DIR.
See <https://bugs.gnu.org/31934> for details.

gnu/packages/linux.scm

@@ -981,15 +981,8 @@ intercept and print the system calls executed by the program.")
                    version ".tar.bz2"))
              (sha256
               (base32
-               "096pwrnhj36yndldvs2pj4r871zhcgisks0is78f1jkjn9sd4b2z"))
-             (patches (search-patches "alsa-lib-add-environment-variable.patch"))))
+               "096pwrnhj36yndldvs2pj4r871zhcgisks0is78f1jkjn9sd4b2z"))))
     (build-system gnu-build-system)
-    (native-search-paths
-     (list (search-path-specification
-             (variable "ALSA_PLUGIN_DIR")
-             (file-type 'regular)
-             (separator #f)                         ;single entry
-             (files '("lib/alsa-lib")))))
     (home-page "https://www.alsa-project.org/")
     (synopsis "The Advanced Linux Sound Architecture libraries")
     (description