net/mlx5e: remove meaningless CFLAGS_tracepoint.o

CFLAGS_tracepoint.o specifies CFLAGS for compiling tracepoint.c but
it does not exist under drivers/net/ethernet/mellanox/mlx5/core/.

CFLAGS_tracepoint.o is unused.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>