Remove setting of GEN_AI_AGENT_NAME from langchain integration

[ericapisani]

Reverts #5030

The agent name is something that we pull from the request, specifically the run_type variable, which is not always the agent name and is semantically incorrect.

This is being removed from on_llm_start as part of a separate set of changes (PR #5705) but this also needs to be removed from the other areas where this is being set in the langchain integration.

[linear-code]

PY-2150