schannel: remove version number and identify its use with 'schannel' literal

Version number is removed in order to make this info consistent with
how we do it with other MS and Linux system libraries for which we don't
provide this info.

Identifier changed from 'WinSSPI' to 'schannel' given that this is the
actual provider of the SSL/TLS support. libcurl can still be built with
SSPI and without SCHANNEL support.