lib/curl_setup.h: remove CURL_WANTS_CA_BUNDLE_ENV

When this define was set, libcurl would check the environment variable
named CURL_CA_BUNDLE at run-time and use that CA cert bundle. This
feature was only defined by the watcom and m32 makefiles and caused
inconsistent behaviours among libcurls built on different platforms.

The curl tool does already feature its own similar logic and the library
does not really need it, and it isn't documented libcurl behavior. So
this change removes it.

Ref: #1538