pkg-config-data.json: Remove m and pthread

These aren't actually pkg-config packages, but packages that
happen to be in cabal2nix's mapping for cabal reasons. They do not
belong in this file.

pkgs/top-level/pkg-config/pkg-config-data.json

@@ -633,9 +633,6 @@
         "zeromq"
       ]
     },
-    "m": {
-      "attrPath": null
-    },
     "menu": {
       "attrPath": [
         "ncurses"
@@ -761,9 +758,6 @@
         "poppler_gi"
       ]
     },
-    "pthread": {
-      "attrPath": null
-    },
     "python": {
       "attrPath": [
         "python3"