Cut down unnecessary lint allows (#14335)

Trying to reduce lint allows either by checking if they are former false
positives or by fixing the underlying warning.

- **Remove dead `allow(dead_code)`**
- **Remove recursive dead code**
- **Remove dead code**
- **Move test only functions to test module**
  The unit tests that use them, themselves are somewhat sus in that they
mock the usage and not test specificly used methods of the
implementation, so there is a risk for divergence
- **Remove `clippy::uninit_vec` allow.**
  May have been a false positive, or the impl has changed somewhat.
We certainly want to look at the unsafe code here to vet for
correctness.

crates/nu-system/src/windows.rs

@@ -471,7 +471,6 @@ unsafe fn null_terminated_wchar_to_string(slice: &[u16]) -> String {
     }
 }
 
-#[allow(clippy::uninit_vec)]
 unsafe fn get_process_data(
     handle: HANDLE,
     ptr: *const c_void,
@@ -518,7 +517,6 @@ unsafe fn get_region_size(handle: HANDLE, ptr: *const c_void) -> Result<usize, &
     Ok((meminfo.RegionSize as isize - ptr.offset_from(meminfo.BaseAddress)) as usize)
 }
 
-#[allow(clippy::uninit_vec)]
 unsafe fn ph_query_process_variable_size(
     process_handle: HANDLE,
     process_information_class: PROCESSINFOCLASS,