net: buf: Remove wrong alignment annotation from NET_BUF_SIMPLE()

The code (net_buf_simple_init in particular) expects the data buffer
to immediately follow the net_buf_simple struct, so it's wrong to
request any specific alignment for this buffer. In practice this
doesn't make any difference since the net_buf_simple struct itself is
4-bytes aligned and a multiple of 4 bytes, however the extra
__net_buf_align makes it look like its location in memory doesn't need
to immediately follow the net_buf_simple struct.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>