refactor: remove needless load_src alias in string_caster (#6093)

`load_src` was a Python 2-era leftover. A removed branch used to reassign
it to a temporary unicode object; since that was deleted with Python 2
support, it is now an exact, never-reassigned alias of `src`. Use `src`
directly.

Spotted in review of #6092.

Assisted-by: ClaudeCode:claude-opus-4.8