apacheGH-36856: [C++] Remove needless braces from BasicDecimal256From…

…LE() arguments (apache#36987)

### Rationale for this change

BasicDecimal256FromLE() requires 4 arguments. So we should not use BasicDecimal256FromLE({X, X, X, X}) form.

I don't know why the current form works on amd64 but doesn't work on s390x.

### What changes are included in this PR?

Removed needless "{" and "}".

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* Closes: apache#36856

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>