[discovery] Fix warnings

[jlaur]

This fixes all warnings in org.openhab.core.config.discovery:

• Remove unused imports and reference explicit namespaces in Javadoc links
• Fix potential null pointer access
• Remove redundant null checks and dead code
• Suppress deprecation warnings in tests (related to Removed deprecated constructor for 'DiscoveryResultImpl' #1428)

One SAT warning remains:

[WARNING] Code Analysis Tool has found:
 0 error(s)!
 1 warning(s)
[WARNING] org.openhab.core.config.discovery.AbstractThingHandlerDiscoveryService.java:[45]
There is no need for a @NonNull annotation because it is set as default. Only @Nullable should be used

openhab-core/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/AbstractThingHandlerDiscoveryService.java

Lines 43 to 45 in 7ea603c

	// this works around a bug in ecj: @NonNullByDefault({}) complains about the field not being
	// initialized when the type is generic, so we have to initialize it with "something"
	protected @NonNullByDefault({}) T thingHandler = (@NonNull T) null;

[J-N-K]

LGTM.