fix: Remove no longer needed code for define-flow-type

Since babel/babel-eslint#449 define-flow-type no
longer needs to check interface declarations to suppress the
no-undef rule.

This commit removes the unneeded code, and moves some test cases from
`VALID_WITH_USE_FLOW_TYPE` to `ALWAYS_VALID` so that the tests pass
again.

src/rules/defineFlowType.js

@@ -49,9 +49,6 @@ const create = (context) => {
         makeDefined(qid);
       }
     },
-    InterfaceDeclaration (node) {
-      makeDefined(node.id);
-    },
     Program () {
       globalScope = context.getScope();
     },

tests/rules/assertions/defineFlowType.js

@@ -91,12 +91,6 @@ const VALID_WITH_DEFINE_FLOW_TYPE = [
       '\'AType\' is not defined.'
     ]
   },
-  {
-    code: 'interface AType {}',
-    errors: [
-      '\'AType\' is not defined.'
-    ]
-  },
   {
     code: '({ a: ({b() {}}: AType) })',
         // `AType` appears twice in `globalScope.through` as distinct
@@ -112,13 +106,6 @@ const VALID_WITH_DEFINE_FLOW_TYPE = [
       '\'AType\' is not defined.',
       '\'BType\' is not defined.'
     ]
-  },
-  {
-    code: 'interface AType<BType> {}',
-    errors: [
-      '\'AType\' is not defined.',
-      '\'BType\' is not defined.'
-    ]
   }
 ];
 
@@ -158,7 +145,9 @@ const ALWAYS_VALID = [
   'function f(a): string {}',
   'class C { a: string }',
   'var AType = {}; class C { a: AType.a }',
-  'declare module A { declare var a: AType }'
+  'declare module A { declare var a: AType }',
+  'interface AType {}',
+  'interface AType<BType> {}'
 ];
 
 /**