[REPL] Do not corrupt input when using numbered_prompt! mode (#59730)
Fix the unbounded possible corruption of the user input by reverting to
a non-scoped begin instead of let.
```julia
julia> ex = Base.remove_linenums!(:(using A; using B; using C)); REPL.Numbered.get_usings!([], ex)
2-element Vector{Any}:
:(using A)
:(using C)
julia> ex
quote
using B
end
```
(cherry picked from commit 05c07e18c3ef2226bc81cb8f22cd1dc2df1defc3)