ena_plat.h: delete WARN_ON dead code

[rlibby]

Summary

gcc issues a "useless statement" error about unlikely(__ret_warn_on); in the WARN_ON in ena_plat.h. In WARN_ONs defined elsewhere, this terminal statement is part of a statement-expression, but not here. Rather than make this WARN_ON like the others, just delete it, as it is apparently unused anyway: the only caller (ENA_ASSERT) always passes false.

This fixes buildkernel for ena-com with gcc.

[rlibby]

Ping. This is a tiny patch for dead code elimination which allows gcc to build ena-com with standard warning flags.