e1000: remove unused and incorrect code

The e1000_clear_vfta function was triggering a warning in kbuild-bot
testing. It's actually a bug but has no functional impact.

drivers/net/ethernet/intel/e1000/e1000_hw.c:4415:58: warning: Same expression in both branches of ternary operator. [duplicateExpressionTernary]

Fix this warning by removing the offending code and simplifying
the routine to do exactly what it did before, no functional
change.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>