ena_plat.h: delete WARN_ON dead code

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.

Submitted by: rlibby
Reviewed by: mw
Approved by: cognet (mentor)