curl_multibyte: Remove local encoding fallbacks

curl_multibyte: Remove local encoding fallbacks

- If the UTF-8 to UTF-16 conversion fails in Windows Unicode builds then
  no longer fall back to assuming the string is in a local encoding.

Background:

Some functions in Windows Unicode builds must convert UTF-8 to UTF-16 to
pass to the Windows CRT API wide-character functions since in Windows
UTF-8 is not a valid locale (or at least 99% of the time right now).

Prior to this change if the Unicode encoding conversion failed then
libcurl would assume, for backwards compatibility with applications that
may have written their code for non-Unicode builds, attempt to convert
the string from local encoding to UTF-16.

That type of "best effort" could theoretically cause some type of
security or other problem if a string that was locally encoded was also
valid UTF-8, and therefore an unexpected UTF-8 to UTF-16 conversion
could occur.

Ref: https://github.com/curl/curl/pull/7246

Closes https://github.com/curl/curl/pull/7257