Remove BodyInserters.fromServerSentEvent variants

Removed superfluous `fromServerSentEvent` variants from `BodyInserters`,
as their functionality can also be obtained by passing a stream of
strings or POJOs (to be encoded as JSON) to
`fromPublisher(Publisher, Class)}`, and specifying a `text/event-stream`
Content-Type.

Issue: SPR-15826

spring-webflux/src/test/java/org/springframework/web/reactive/function/BodyInsertersTests.java

@@ -247,17 +247,6 @@ public class BodyInsertersTests {
 		StepVerifier.create(result).expectNextCount(0).expectComplete().verify();
 	}
 
-	@Test
-	public void ofServerSentEventClass() throws Exception {
-		Flux<String> body = Flux.just("foo");
-		BodyInserter<Flux<String>, ServerHttpResponse> inserter =
-				BodyInserters.fromServerSentEvents(body, String.class);
-
-		MockServerHttpResponse response = new MockServerHttpResponse();
-		Mono<Void> result = inserter.insert(response, this.context);
-		StepVerifier.create(result).expectNextCount(0).expectComplete().verify();
-	}
-
 	@Test
 	public void ofFormData() throws Exception {
 		MultiValueMap<String, String> body = new LinkedMultiValueMap<>();

spring-webflux/src/test/java/org/springframework/web/reactive/function/server/SseHandlerFunctionIntegrationTests.java

@@ -25,6 +25,7 @@ import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
 
 import org.springframework.core.ParameterizedTypeReference;
+import org.springframework.http.MediaType;
 import org.springframework.http.codec.ServerSentEvent;
 import org.springframework.web.reactive.function.client.WebClient;
 
@@ -120,13 +121,17 @@ public class SseHandlerFunctionIntegrationTests extends AbstractRouterFunctionIn
 
 		public Mono<ServerResponse> string(ServerRequest request) {
 			Flux<String> flux = Flux.interval(Duration.ofMillis(100)).map(l -> "foo " + l).take(2);
-			return ServerResponse.ok().body(fromServerSentEvents(flux, String.class));
+			return ServerResponse.ok()
+					.contentType(MediaType.TEXT_EVENT_STREAM)
+					.body(flux, String.class);
 		}
 
 		public Mono<ServerResponse> person(ServerRequest request) {
 			Flux<Person> flux = Flux.interval(Duration.ofMillis(100))
 					.map(l -> new Person("foo " + l)).take(2);
-			return ServerResponse.ok().body(fromServerSentEvents(flux, Person.class));
+			return ServerResponse.ok()
+					.contentType(MediaType.TEXT_EVENT_STREAM)
+					.body(flux, Person.class);
 		}
 
 		public Mono<ServerResponse> sse(ServerRequest request) {