[intl] unbreak build with ICU 57

Remove a call to `icu::toUCharPtr()` that wasn't present in other
similar looking call sites either, just reinterpret_cast directly.

Fixes https://github.com/nodejs/node/issues/19656.

Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: If281ce0a39356aa8bd20efb24c3e4b52b06841a3
Reviewed-on: https://chromium-review.googlesource.com/987953Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Commit-Queue: Ben Noordhuis <info@bnoordhuis.nl>
Cr-Commit-Position: refs/heads/master@{#52311}

src/runtime/runtime-intl.cc

@@ -620,8 +620,7 @@ RUNTIME_FUNCTION(Runtime_PluralRulesSelect) {
   icu::UnicodeString result = plural_rules->select(rounded);
   return *isolate->factory()
               ->NewStringFromTwoByte(Vector<const uint16_t>(
-                  reinterpret_cast<const uint16_t*>(
-                      icu::toUCharPtr(result.getBuffer())),
+                  reinterpret_cast<const uint16_t*>(result.getBuffer()),
                   result.length()))
               .ToHandleChecked();
 }