bnxt_en: Remove BNXT_FLAG_USING_MSIX flag

bnxt_en: Remove BNXT_FLAG_USING_MSIX flag

Now that we only support MSIX, the BNXT_FLAG_USING_MSIX is always true.
Remove it and any if conditions checking for it.  Remove the INTX
handler and associated logic.

Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Reviewed-by: Hongguang Gao <hongguang.gao@broadcom.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Link: https://patch.msgid.link/20240828183235.128948-6-michael.chan@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>