Drop support of Botan without TLS Boost

Currently a degraded version of Kea can be built with Botan as the crypto backend (vs OpenSSL) but without the TLS Boost feature. It does not make sense in the current Internet so I propose to drop the support of this corner case and update configure (ax_crypto.m4) raises an error (vs warning) when Botan was not installed with TLS Boost (--with-boost in Botan config).