hw/xen: Remove unnecessary xen_hvm_inject_msi() stub

Since commit 04b0de0ee8 ("xen: factor out common functions")
xen_hvm_inject_msi() stub is not required.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <20231114143816.71079-8-philmd@linaro.org>

stubs/xen-hw-stub.c

@@ -24,10 +24,6 @@ int xen_set_pci_link_route(uint8_t link, uint8_t irq)
     return -1;
 }
 
-void xen_hvm_inject_msi(uint64_t addr, uint32_t data)
-{
-}
-
 int xen_is_pirq_msi(uint32_t msi_data)
 {
     return 0;