Replace obsolete @NullableDecl

[olekw]

Also, remove dependency on checker-compat-qual since it is no longer needed.
From downstream fix in Debian.

Fix for #497

[cpovirk]

Thanks. Some of these changes produce different meanings for the annotations, as laid out in https://checkerframework.org/manual/#faq-array-syntax-meaning and https://checkerframework.org/manual/#declaration-annotations-moved. I've started tweaking a copy of this PR to try to make that work, but I've hit a tool bug that I'm reporting.

It's possible that the right solution is for us to give up on nullness annotations entirely (at least in this open-source copy of the library) until we can require our users to use Java 8. (Maybe we could get away with requiring Java 8 even today: Even though some of our users target Java 7, they are likely all testing with Java 8+.) Maybe we'll remove them from our external copy entirely, or maybe we'll use Retrolambda to rewrite Java 8 bytecode to Java 7 bytecode (but solely for the purpose of removing type annotations, without actually touching lambdas).

(Or we could migrate to a different system of nullness annotations, one that still uses declaration annotations.)

I will figure something out, but it's looking like it may take some time.

[cpovirk]

Update: Someone added a workaround for us. So now I've hit... another bug in another tool! And a diamond-dependency conflict! I'll keep plugging away....