global_init: ignore CURL_GLOBAL_SSL's absense

This bit is no longer used. It is not clear what it meant for users to
"init the TLS" in a world with different TLS backends and since the
introduction of multissl, libcurl didn't properly work if inited without
this bit set.

Not a single user responded to the call for users of it:
https://curl.haxx.se/mail/lib-2017-11/0072.html

Reported-by: Evgeny Grin
Assisted-by: Jay Satiro

Fixes #2089
Fixes #2083
Closes #2107