core: Remove temporary AbstractServerImplBuilder

This breaks the ABI of the classes listed below.

Users that recompiled  their code using grpc-java [`v1.36.0`]
(https://github.com/grpc/grpc-java/releases/tag/v1.36.0) (released on
Feb 23, 2021) and later, ARE NOT AFFECTED.

Users that compiled their source using grpc-java earlier than
[`v1.36.0`]
(https://github.com/grpc/grpc-java/releases/tag/v1.36.0) need to
recompile when upgrading to grpc-java `v1.59.0`. Otherwise the code
will fail on runtime with `NoSuchMethodError`. For example, code:

```java
NettyServerBuilder.forPort(80).directExecutor();
```

Will fail with

> `java.lang.NoSuchMethodError: 'io.grpc.internal.AbstractServerImplBuilder
  io.grpc.netty.NettyServerBuilder.directExecutor()'`

**Affected classes**

Class `AbstractServerImplBuilder` is deleted, and no longer in the
class hierarchy of the server builders:
- `io.grpc.netty.NettyServerBuilder`
- `io.grpc.inprocess.InProcessServerBuilder`

core/build.gradle

@@ -113,11 +113,6 @@ plugins.withId("java") {
                     "io/grpc/internal/AbstractManagedChannelImplBuilder.class").get().getAsFile(),
                 "<T:Lio/grpc/internal/AbstractManagedChannelImplBuilder<TT;>;>Lio/grpc/ManagedChannelBuilder<TT;>;",
                 "<T:Lio/grpc/ManagedChannelBuilder<TT;>;>Lio/grpc/ManagedChannelBuilder<TT;>;");
-            project.replaceConstant(
-                destinationDirectory.file(
-                    "io/grpc/internal/AbstractServerImplBuilder.class").get().getAsFile(),
-                "<T:Lio/grpc/internal/AbstractServerImplBuilder<TT;>;>Lio/grpc/ServerBuilder<TT;>;",
-                "<T:Lio/grpc/ServerBuilder<TT;>;>Lio/grpc/ServerBuilder<TT;>;");
         }
     }
 }

inprocess/src/main/java/io/grpc/inprocess/InProcessServerBuilder.java

@@ -22,10 +22,10 @@
 import com.google.errorprone.annotations.DoNotCall;
 import io.grpc.Deadline;
 import io.grpc.ExperimentalApi;
+import io.grpc.ForwardingServerBuilder;
 import io.grpc.Internal;
 import io.grpc.ServerBuilder;
 import io.grpc.ServerStreamTracer;
-import io.grpc.internal.AbstractServerImplBuilder;
 import io.grpc.internal.FixedObjectPool;
 import io.grpc.internal.GrpcUtil;
 import io.grpc.internal.InternalServer;
@@ -73,8 +73,7 @@
  * </pre>
  */
 @ExperimentalApi("https://github.com/grpc/grpc-java/issues/1783")
-public final class InProcessServerBuilder extends
-    AbstractServerImplBuilder<InProcessServerBuilder> {
+public final class InProcessServerBuilder extends ForwardingServerBuilder<InProcessServerBuilder> {
   /**
    * Create a server builder that will bind with the given name.
    *

netty/src/main/java/io/grpc/netty/NettyServerBuilder.java

@@ -30,11 +30,11 @@
 import com.google.errorprone.annotations.InlineMe;
 import io.grpc.Attributes;
 import io.grpc.ExperimentalApi;
+import io.grpc.ForwardingServerBuilder;
 import io.grpc.Internal;
 import io.grpc.ServerBuilder;
 import io.grpc.ServerCredentials;
 import io.grpc.ServerStreamTracer;
-import io.grpc.internal.AbstractServerImplBuilder;
 import io.grpc.internal.FixedObjectPool;
 import io.grpc.internal.GrpcUtil;
 import io.grpc.internal.InternalServer;
@@ -67,7 +67,7 @@
  */
 @ExperimentalApi("https://github.com/grpc/grpc-java/issues/1784")
 @CheckReturnValue
-public final class NettyServerBuilder extends AbstractServerImplBuilder<NettyServerBuilder> {
+public final class NettyServerBuilder extends ForwardingServerBuilder<NettyServerBuilder> {
 
   // 1MiB
   public static final int DEFAULT_FLOW_CONTROL_WINDOW = 1024 * 1024;