Remove custom lowdown

This was last upgraded in 788008385e, but
the version in Nixpkgs is a now a lot newer. I think the custom was
added to get ahead of Nixpkgs before, and so now that we are in fact
behind, it is no longer needed.

flake.lock

@@ -32,22 +32,6 @@
         "type": "github"
       }
     },
-    "lowdown-src": {
-      "flake": false,
-      "locked": {
-        "lastModified": 1633514407,
-        "narHash": "sha256-Dw32tiMjdK9t3ETl5fzGrutQTzh2rufgZV4A/BbxuD4=",
-        "owner": "kristapsdz",
-        "repo": "lowdown",
-        "rev": "d2c2b44ff6c27b936ec27358a2653caaef8f73b8",
-        "type": "github"
-      },
-      "original": {
-        "owner": "kristapsdz",
-        "repo": "lowdown",
-        "type": "github"
-      }
-    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1701355166,
@@ -84,7 +68,6 @@
       "inputs": {
         "flake-compat": "flake-compat",
         "libgit2": "libgit2",
-        "lowdown-src": "lowdown-src",
         "nixpkgs": "nixpkgs",
         "nixpkgs-regression": "nixpkgs-regression"
       }