fix jl_eqtable_del and use delete-with-default in reload_path.

    julia> using Example
    in delete!: val not defined

@carlobaldassi's fix avoids this case, which fixes the immediate
problem, but the correct behavior here is to throw a key error:

    julia> using Example
    key not found: SOURCE_PATH

With a corrected jl_eqtable_del it's more elegant to write this:

    prev = delete!(tls, :SOURCE_PATH, nothing)