src: remove unused agent_ in node_trace_buffer

Currently the following compiler warning is generated:

In file included from ../src/tracing/node_trace_buffer.cc:1:
../src/tracing/node_trace_buffer.h:75:10:
warning: private field 'agent_' is not used [-Wunused-private-field]
  Agent* agent_;
         ^
1 warning generated.

This commit removes the private member from the header and also from the constructors initializer list.

I was not sure if it should be removed completely from the constructor or not, but hopefully someone can shed some light on whether it should.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines