packaging: remove obsolete boost-finding environment variables

The issue requiring these environment variables to be set for Meson to find
Boost [1] has been fixed [2] for quite some time now. Drop them since they are
unnecessary in all supported nixpkgs releases (in fact they have already been
removed in the lix package there).

[1] https://github.com/NixOS/nixpkgs/issues/86131
[2] https://github.com/NixOS/nixpkgs/pull/315998

Change-Id: I6a6a69640a30e917cd7a983b9d846d023b393dcd

package.nix

@@ -377,11 +377,6 @@ stdenv.mkDerivation (finalAttrs: {
   ];
 
   env = {
-    # Needed for Meson to find Boost.
-    # https://github.com/NixOS/nixpkgs/issues/86131.
-    BOOST_INCLUDEDIR = "${lib.getDev boost}/include";
-    BOOST_LIBRARYDIR = "${lib.getLib boost}/lib";
-
     # Meson allows referencing a /usr/share/cargo/registry shaped thing for subproject sources.
     # Turns out the Nix-generated Cargo dependencies are named the same as they
     # would be in a Cargo registry cache.