CG-10705: API to remove async from functions

[codegen-sh]

This PR adds a new API method remove_async() to the Function class that allows removing the async keyword from functions.

Changes

• Added a new method remove_async() to the Function class in src/codegen/sdk/core/function.py
• The method checks if a function is async and removes the async keyword if present
• The method handles whitespace properly to maintain code formatting

Usage

# Find an async function
async_function = codebase.find_function("my_async_function")

# Remove the async keyword
async_function.remove_async()

This complements the existing asyncify() method which adds the async keyword to functions.

Resolves CG-10705