Remove redundant "usage" metadata

[sleepwithcoffee]

Issue #, if available:

#620

Description of changes:

Resolve the issue

Additional kwargs key total_tokens already exists in left dict and value has unsupported type <class 'decimal.Decimal'>.

After removing, "usage" metadata is still being tracked in succession

As an alternative, I tried upgrading langchain and its dependencies, the issue still persists after that.