NetworkPkg/HttpDxe: drop misleading comment / status code in cert config

For TlsConfigureSession(), it makes sense to exempt EFI_NOT_FOUND from
TlsConfigCipherList() / gRT->GetVariable(), because there is a default
cipher list (SSL_DEFAULT_CIPHER_LIST) we can fall back to.

The same is not true of TlsConfigCertificate(), because there is no
default CA cert list. The platform (or the user of the Setup utility) is
required to configure a CA cert list first.

Remove the misleading comment and status code mapping in
TlsConfigCertificate().

Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>