Remove findbugs

[iamdanfox]

The findbugs plugin was removed from baseline in March 2017 (palantir/gradle-baseline#131), so this repo contains many (unvalidated) findbugs annotations.

These would be mostly harmless, except that they cause a new com.palantir.baseline-class-uniqueness plugin to fail: the findbugs jars are notoriously unhygienic. jsr305, annotations and findbugs-annotations all define some classes with the same name.

Rather than just ignoring these violations, I figured we might as well fix them at the source :)

[schlosna]

For future reference, worth tracking and investigating:

• Consider supporting JSR 305 annotations google/error-prone#44
• Create more annotations like @ParametersAreNonnullByDefault google/error-prone#280
• https://github.com/uber/NullAway
• https://checkerframework.org/