qemu-riscv: Remove obsolete CSR - send_ipi

This aligns the code in qemu-riscv with the code in spike-riscv.
The previous code gives an error in the updated toolchain as the
send_ipi CSR is no longer valid.

This gave the build error:
src/mainboard/emulation/qemu-riscv/qemu_util.c:64:
   Error: Instruction csrw requires absolute expression

Change-Id: Iac0f66e8e9935f45c8094d5e16bedb7ac5225424
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/16244
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>

src/mainboard/emulation/qemu-riscv/qemu_util.c

@@ -56,14 +56,7 @@ uintptr_t mcall_query_memory(uintptr_t id, memory_block_info *p)
 
 uintptr_t mcall_send_ipi(uintptr_t recipient)
 {
-	//if (recipient >= num_harts)
-	//return -1;
-
-	if (atomic_swap(&OTHER_HLS(recipient)->ipi_pending, 1) == 0) {
-		mb();
-		write_csr(send_ipi, recipient);
-	}
-
+	die("mcall_send_ipi is currently not implemented");
 	return 0;
 }