Revert #255422: libsodium: 1.0.18 -> 1.0.19

This reverts commit 16ed72e9b548a35511061fb89929fce309e44288, reversing
changes made to 8530ff3965aae9ecfeb4410e0b3f775e9aa6fd4d.

-Ofast in default flags sounds like a bad idea,
and it also breaks some of our builds. Upstream:
https://github.com/jedisct1/libsodium/commit/ad4584d45#commitcomment-129441988
- I see no change that we'd need to have now:
https://github.com/jedisct1/libsodium/blob/1.0.19/ChangeLog

pkgs/development/libraries/libsodium/default.nix

···
 
 stdenv.mkDerivation (finalAttrs: {
   pname = "libsodium";
-  version = "1.0.19";
+  version = "1.0.18";
 
   src = fetchurl {
     url = "https://download.libsodium.org/libsodium/releases/${finalAttrs.pname}-${finalAttrs.version}.tar.gz";
-    sha256 = "sha256-AY15/goEXMoHMx03vQy1ey6DjFG8SP2DehRy5QBou+o=";
+    sha256 = "1h9ncvj23qbbni958knzsli8dvybcswcjbx0qjjgi922nf848l3g";
   };
 
   outputs = [ "out" "dev" ];