Retire our "use-LC_NUMERIC-internally" workaround

Commit 8dc3982408 (Always use LC_NUMERIC=C internally (#8204),
2021-10-13) made us use LC_NUMERIC=C internally, to make C library
functions behave in a predictable way.

We no longer use library functions affected by LC_NUMERIC[*]..

Since the effective value of LC_NUMERIC no longer matters, let's
simplify things by using the user locale again, like we do for the
other locale variables.

The printf crate still uses libc::snprintf() which respects LC_NUMERIC,
but it looks like "cargo test" creates a separate process per crate,
and the printf crate does not call setlocale().

src/env_dispatch.rs

@@ -534,11 +534,6 @@ fn init_locale(vars: &EnvStack) {
         loc
     };
 
-    // We *always* use a C-locale for numbers because we want '.' (except for in printf).
-    let loc_ptr = unsafe { libc::setlocale(libc::LC_NUMERIC, c"C".as_ptr()) };
-    // should never fail, the C locale should always be defined
-    assert_ne!(loc_ptr, ptr::null_mut());
-
     // Update cached locale information.
     crate::common::fish_setlocale();
     FLOG!(

src/locale.rs

@@ -108,18 +108,8 @@ unsafe fn read_locale() -> Option<Locale> {
     // Bleh, we have to go through localeconv, which races with setlocale.
     // TODO: There has to be a better way to do this.
     let _guard = LOCALE_LOCK.lock().unwrap();
-
-    unsafe {
-        libc::setlocale(libc::LC_NUMERIC, c"".as_ptr());
-    }
-
     let lconv = unsafe { libc::localeconv() };
-    let result = (!lconv.is_null()).then(|| unsafe { lconv_to_locale(&*lconv) });
-    // Note we *always* use a C-locale for numbers, because we always want "." except for in printf.
-    unsafe {
-        libc::setlocale(libc::LC_NUMERIC, c"C".as_ptr());
-    }
-    result
+    (!lconv.is_null()).then(|| unsafe { lconv_to_locale(&*lconv) })
 }
 
 // Current numeric locale.