feat(datasource/custom): remove content limiter for plain (#29549)

lib/modules/datasource/custom/formats/plain.ts

@@ -20,11 +20,6 @@ export class PlainFetcher implements CustomDatasourceFetcher {
   async fetch(http: Http, registryURL: string): Promise<unknown> {
     const response = await http.getPlain(registryURL);
 
-    const contentType = response.headers['content-type'];
-    if (!contentType?.startsWith('text/')) {
-      return null;
-    }
-
     return convertLinesToVersions(response.body);
   }
 

lib/modules/datasource/custom/index.spec.ts

@@ -209,30 +209,6 @@ describe('modules/datasource/custom/index', () => {
       expect(result).toEqual(expected);
     });
 
-    it('return null for plain text API if the body is not what is expected', async () => {
-      const expected = {
-        releases: [
-          {
-            version: '1.0.0',
-          },
-        ],
-      };
-      httpMock.scope('https://example.com').get('/v1').reply(200, expected, {
-        'Content-Type': 'application/json',
-      });
-      const result = await getPkgReleases({
-        datasource: `${CustomDatasource.id}.foo`,
-        packageName: 'myPackage',
-        customDatasources: {
-          foo: {
-            defaultRegistryUrlTemplate: 'https://example.com/v1',
-            format: 'plain',
-          },
-        },
-      });
-      expect(result).toBeNull();
-    });
-
     it('return releases for yaml API directly exposing in Renovate format', async () => {
       const expected = {
         releases: [