No global settings in libnixfetchers and libnixflake

Progress on #5638

There are still a global fetcher and eval settings, but they are pushed
down into `libnixcmd`, which is a lot less bad a place for this sort of
thing.

Continuing process pioneered in
52bfccf8d8.

src/libfetchers/fetch-settings.cc

@@ -1,14 +1,9 @@
 #include "fetch-settings.hh"
-#include "config-global.hh"
 
-namespace nix {
+namespace nix::fetchers {
 
-FetchSettings::FetchSettings()
+Settings::Settings()
 {
 }
 
-FetchSettings fetchSettings;
-
-static GlobalConfig::Register rFetchSettings(&fetchSettings);
-
 }