Remove tls_ca_certificates variable

`tls_ca_certificates` is only used once and it may be that it is not
even used depending on the `if` condition. Instead of assign it, we can
just call the method when it is needed.

(cherry picked from commit 0a41598)

lib/bunny/transport.rb

@@ -357,7 +357,6 @@ def prepare_tls_context(opts)
       @tls_key               = tls_key_from(opts)
       @tls_certificate_store = opts[:tls_certificate_store]
 
-      @tls_ca_certificates   = tls_ca_certificates_paths_from(opts)
       @verify_peer           = as_boolean(opts[:verify_ssl] || opts[:verify_peer] || opts[:verify])
 
       @tls_context = initialize_tls_context(OpenSSL::SSL::SSLContext.new, opts)
@@ -416,7 +415,7 @@ def initialize_tls_context(ctx, opts={})
       ctx.cert_store = if @tls_certificate_store
                          @tls_certificate_store
                        else
-                         initialize_tls_certificate_store(@tls_ca_certificates)
+                         initialize_tls_certificate_store(tls_ca_certificates_paths_from(opts))
                        end
 
       if !@tls_certificate