[s390] Remove stray BUILTIN_CALL_TRIPLE reference

Missed from the earlier ObjectTriple removal, commit fc413158.

Bug: 
Change-Id: I2fd9c17b4a4d888d81dc0b51586bec6f191cc7ed
Reviewed-on: https://chromium-review.googlesource.com/707138Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#48392}

src/s390/simulator-s390.cc

@@ -1976,9 +1976,8 @@ void Simulator::SoftwareInterrupt(Instruction* instr) {
       int arg0_regnum = 2;
       intptr_t result_buffer = 0;
       bool uses_result_buffer =
-          redirection->type() == ExternalReference::BUILTIN_CALL_TRIPLE ||
-          (redirection->type() == ExternalReference::BUILTIN_CALL_PAIR &&
-           !ABI_RETURNS_OBJECTPAIR_IN_REGS);
+          redirection->type() == ExternalReference::BUILTIN_CALL_PAIR &&
+          !ABI_RETURNS_OBJECTPAIR_IN_REGS;
       if (uses_result_buffer) {
         result_buffer = get_register(r2);
         arg0_regnum++;