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 76e047fc27#7540
Assisted-by: Jay Satiro
Closes#17843
