Remove `ABSL_NULLABILITY_COMPATIBLE` from `{,Stable}Dependency`.

They are normally used in templates as `Dependency<Handle, Manager>`, with
`Manager` depending on the template parameter. Their nullability should reflect
the nullability of `Manager`, and even the nullability of `Handle`, which is
normally fixed when `Dependency` is used, should reflect the nullability of
`Manager`. This cannot be expressed.

Furthermore, even if it could be expressed, it would not propagate further.
For example in `StringWriter<std::string* absl_nonnull>`, `DestString()`
should return `std::string* absl_nonnull`, and in
`StringWriter<std::string* absl_nullable>`, `DestString()` should return
`std::string* absl_nullable`. But `DestString()` is a virtual method in
non-templated `StringWriterBase`, so its nullability cannot depend on the
template instantiation of `StringWriter`.

Furthermore, even if it propagated further, it would not express conditional
nullability. Even if `StringWriterBase::DestString()` could return
`std::string* absl_nullable`, the result can be `nullptr` only if the
`StringWriterBase` is closed. Methods which call `*DestString()` do not reach
this call when the `StringWriterBase` is closed, which is why they can
dereference the result.

OTOH in `Dependency<Handle, Manager>`, nullability of `Manager` already
propagates to some other public methods. For example in
`StringWriter<std::string* absl_nonnull>`, `dest()` returns
`std::string* absl_nonnull&`, and in `StringWriter<std::string* absl_nullable>`,
`dest()` returns `std::string* absl_nullable&`. For that it is not needed to
make `Dependency` itself nullable.

PiperOrigin-RevId: 802531307