Remove redundant else in activation.py

[leaderofARS]

Remove Redundant Else Block in get_activation()

Fixes #42621

This PR removes a redundant else block in get_activation() in src/transformers/activations.py. The else was unnecessary because the if branch already returned, and keeping it went against the project's preferred early-return style.

Previous Control Flow

def get_activation(activation_string):
    if activation_string in ACT2FN:
        return ACT2FN[activation_string]
    else:
        raise KeyError(f"function {activation_string} not found in ACT2FN mapping {list(ACT2FN.keys())}")

Updated Control Flow

def get_activation(activation_string):
    if activation_string in ACT2FN:
        return ACT2FN[activation_string]
    raise KeyError(f"function {activation_string} not found in ACT2FN mapping {list(ACT2FN.keys())}")

Summary

• Pure cleanup (no behavior change)
• Simplifies logic and matches HF style
• All style & repo checks pass