vtls: Fixed compilation warning and an ignored return code

curl_schannel.h:123: warning: right-hand operand of comma expression
                     has no effect

Some instances of the curlssl_close_all() function were declared with a
void return type whilst others as int. The schannel version returned
CURLE_NOT_BUILT_IN and others simply returned zero, but in all cases the
return code was ignored by the calling function Curl_ssl_close_all().

For the time being and to keep the internal API consistent, changed all
declarations to use a void return type.

To reduce code we might want to consider removing the unimplemented
versions and use a void #define like schannel does.