[BUG][JAVA][okhttp-gson] Remove test stubs from FILES index

CI's bin/generate-samples.sh regen skips writing test files when they
already exist, and only newly-written files go into the FILES index.
The test stubs were emitted on first regen and listed in FILES; CI's
re-regen sees them existing -> skips -> filters them from FILES,
producing a 'Samples up-to-date' diff. Remove the four NullableFields*
test stub entries to match what regen actually produces.

Author: winrid

samples/client/petstore/java/okhttp-gson/.openapi-generator/FILES

@@ -277,7 +277,3 @@ src/main/java/org/openapitools/client/model/Value.java
 src/main/java/org/openapitools/client/model/Variable.java
 src/main/java/org/openapitools/client/model/Whale.java
 src/main/java/org/openapitools/client/model/Zebra.java
-src/test/java/org/openapitools/client/model/NullableFieldsMapErrorTest.java
-src/test/java/org/openapitools/client/model/NullableFieldsMapResponseTest.java
-src/test/java/org/openapitools/client/model/NullableFieldsMapSuccessTest.java
-src/test/java/org/openapitools/client/model/NullableFieldsValueTest.java