vtls: Removed unimplemented overrides of curlssl_close_all()

Carrying on from commit 037cd0d9, 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()