Remove old ddtrace require from tracing benchmarks

The library has been renamed to datadog,
this appears to be the last location where ddtrace
is required in the test suite.

benchmarks/tracing_trace.rb

@@ -5,13 +5,7 @@
 
 require 'benchmark/ips'
 require 'open3'
-
-begin
-  require 'datadog'
-rescue LoadError # TODO: Remove when ddtrace 2.0 is merged to master
-  # This is required to run benchmarks against ddtrace 1.x.
-  require 'ddtrace'
-end
+require 'datadog'
 
 class TracingTraceBenchmark
   module NoopWriter