Bluetooth: tests: fix `buf` tests

Delete tests that did not end up bringing any value.

What ended up happening is busy-work to "make the test pass" without
understanding what's their original purpose.

Worse, the CI change-based testing is broken and doesn't pick them up,
even by PRs modifying the tests themeselves.
See #68008

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>