Remove unused flatten_literal_params function

[Copilot]

The flatten_literal_params function in discord/utils.py became dead code after removing Python < 3.10 compatibility. It was previously called conditionally in evaluate_annotation to flatten nested Literal types on older Python versions:

# Before (in master):
if tp.__origin__ is Literal:
    if not PY_310:
        args = flatten_literal_params(tp.__args__)  # Only for Python < 3.10
    
# After PR #2988:
if tp.__origin__ is Literal:
    # flatten_literal_params no longer called

Changes

• Removed flatten_literal_params function (11 lines) from discord/utils.py
• No other code references this function

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[pycord-app]

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