inspector: remove AgentImpl

AgentImpl was introduce so inspector-agent.h does not leak libuv and
inspector implementation details. Inspector had been reworked since and
new classes provide this isolation and AgentImpl became unnecessary
level of indirection. This change removes that class to streamline the
code.

PR-URL: https://github.com/nodejs/node/pull/12576


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>