windows: drop redundant `curl_wcsdup_callback` callback

windows: drop redundant `curl_wcsdup_callback` callback

This callback was permanently mapped to libcurl's internal
`Curl_wcsdup()`, which always uses the customizable malloc for
allocation, thus making a custom mapping redundant anyway.

To simplify, drop the callback and map `_tcsdup()` in Unicode mode
directly to `Curl_wcsdup()`.

Also fixes:
- `curl_global_init()` which, before this patch, (re)initialized its
  mapping to `_wcsdup()`, returning buffers potentially incompatible
  with a custom allocator.
  Bug: https://github.com/curl/curl/pull/17840#issuecomment-3044361245
  Bug: https://github.com/curl/curl/pull/7540#issuecomment-2380995349
  Co-reported-by: Luca Kellermann
Follow-up to 76e047fc27b3a0b9e6d6d00cacf536e7b7c1b532 #7540
Assisted-by: Jay Satiro
Closes #17843