genirq/msi: Drop dead domain name assignment

Since commit d59f6617 ("genirq: Allow fwnode to carry name
information only") an IRQ domain is always given a name during
allocation (e.g. used for the debugfs entry).

Drop the unused fallback name assignment when creating MSI domains.

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230224130509.27814-1-johan+linaro@kernel.org