refactor: remove call to BuildNameToCertificate (deprecated)

From https://golang.org/doc/go1.14:

When multiple certificate chains are provided through the Config.Certificates field, the first one compatible with the peer is now automatically selected. This allows for example providing an ECDSA and an RSA certificate, and letting the package automatically select the best one. Note that the performance of this selection is going to be poor unless the Certificate.Leaf field is set. The Config.NameToCertificate field, which only supports associating a single certificate with a give name, is now deprecated and should be left as nil. Similarly the Config.BuildNameToCertificate method, which builds the NameToCertificate field from the leaf certificates, is now deprecated and should not be called.