treewide: remove dummy Makefiles for single targets

Now that the single target build descends into sub-directories in the
same way as the normal build, these dummy Makefiles are not needed
any more.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

drivers/net/ethernet/aquantia/atlantic/hw_atl/Makefile

-# SPDX-License-Identifier: GPL-2.0
-# kbuild requires Makefile in a directory to build individual objects

drivers/net/ethernet/mellanox/mlx5/core/accel/Makefile

-# SPDX-License-Identifier: GPL-2.0-only
-subdir-ccflags-y += -I$(src)/..

drivers/net/ethernet/mellanox/mlx5/core/diag/Makefile

-# SPDX-License-Identifier: GPL-2.0-only
-subdir-ccflags-y += -I$(src)/..

drivers/net/ethernet/mellanox/mlx5/core/en/Makefile

-# SPDX-License-Identifier: GPL-2.0-only
-subdir-ccflags-y += -I$(src)/..

drivers/net/ethernet/mellanox/mlx5/core/en/xsk/Makefile

-subdir-ccflags-y += -I$(src)/../..

drivers/net/ethernet/mellanox/mlx5/core/en_accel/Makefile

-# SPDX-License-Identifier: GPL-2.0-only
-subdir-ccflags-y += -I$(src)/..

drivers/net/ethernet/mellanox/mlx5/core/fpga/Makefile

-# SPDX-License-Identifier: GPL-2.0-only
-subdir-ccflags-y += -I$(src)/..

drivers/net/ethernet/mellanox/mlx5/core/ipoib/Makefile

-# SPDX-License-Identifier: GPL-2.0-only
-subdir-ccflags-y += -I$(src)/..

drivers/net/ethernet/mellanox/mlx5/core/lib/Makefile

-# SPDX-License-Identifier: GPL-2.0-only
-subdir-ccflags-y += -I$(src)/..

drivers/net/ethernet/netronome/nfp/bpf/Makefile

-# SPDX-License-Identifier: GPL-2.0
-# kbuild requires Makefile in a directory to build individual objects

drivers/net/ethernet/netronome/nfp/flower/Makefile

-# SPDX-License-Identifier: GPL-2.0
-# kbuild requires Makefile in a directory to build individual objects

drivers/net/ethernet/netronome/nfp/nfpcore/Makefile

-# SPDX-License-Identifier: GPL-2.0
-# kbuild requires Makefile in a directory to build individual objects

drivers/net/ethernet/netronome/nfp/nfpcore/nfp6000/Makefile

-# SPDX-License-Identifier: GPL-2.0
-# kbuild requires Makefile in a directory to build individual objects

drivers/net/ethernet/netronome/nfp/nic/Makefile

-# SPDX-License-Identifier: GPL-2.0
-# kbuild requires Makefile in a directory to build individual objects