Carrying on from commit 037cd0d991, removed the following unimplemented
instances of curlssl_close_all():
Curl_axtls_close_all()
Curl_darwinssl_close_all()
Curl_cyassl_close_all()
Curl_gskit_close_all()
Curl_gtls_close_all()
Curl_nss_close_all()
Curl_polarssl_close_all()
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.
