ice: Remove unnecessary flag ICE_FLAG_MSIX_ENA

This flag is not needed and is called every time we re-enable interrupts
in the hotpath so remove it. Also remove ice_vsi_req_irq() because it
was a wrapper function for ice_vsi_req_irq_msix() whose sole purpose was
checking the ICE_FLAG_MSIX_ENA flag.

Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>