Remove the CURL_CA_FALLBACK logic. That build option was added to allow
primarily OpenSSL to use the default paths for loading the CA certs. For
GnuTLS it was instead made to load the "system certs", which is
different and not desirable.
The native CA store loading is now asked for with this option.
Follow-up to 7b55279d1d
Co-authored-by: Jay Satiro
Closes#12137