doc: Update gpgrt.texi for old gpg-error-config script.

* doc/gpgrt.texi: Address pkg-config.  Manual entry is for
gpg-error-config.  gpgrt-config is an internal command for
gpg-error.m4.

Remove about variable gpg_config_script_warn, which is only
relevant to a use case of old gpg-error-config.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>