tests: Remove broken net_buf unit tests

The net_buf unit tests were doing some pretty hacky stuff with
defining a dummy _net_buf_pool_list variable to fulfil the linker
magic that happens with a real build. Until now the code got lucky in
that the simple net_buf_alloc() tests didn't depend on
_net_buf_pool_list being valid, however with the coming redesign even
net_buf_alloc will require proper setup of this.

Since the unit tests were extremely minimal, and not testing anything
beyond what tests/net/buf already tests, just remove them for now.

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