remove polling remote execution client (#10306)

### Problem

As described in #10270, we should remove the "polling" remote execution client because it has been superseded by the "streaming" remote execution client.

### Solution

Remove the polling remote execution client.

### Result

It is gone.

Implements #10270

src/python/pants/engine/internals/native.py

@@ -222,7 +222,6 @@ def new_scheduler(
             execution_headers=tuple(
                 (k, v) for (k, v) in execution_options.remote_execution_headers.items()
             ),
-            execution_enable_streaming=execution_options.remote_execution_enable_streaming,
             execution_overall_deadline_secs=execution_options.remote_execution_overall_deadline_secs,
         )
 

src/python/pants/option/global_options.py

@@ -84,7 +84,6 @@ class ExecutionOptions:
     remote_oauth_bearer_token_path: Any
     remote_execution_extra_platform_properties: Any
     remote_execution_headers: Any
-    remote_execution_enable_streaming: bool
     remote_execution_overall_deadline_secs: int
     process_execution_local_enable_nailgun: bool
 
@@ -111,7 +110,6 @@ def from_bootstrap_options(cls, bootstrap_options):
             remote_oauth_bearer_token_path=bootstrap_options.remote_oauth_bearer_token_path,
             remote_execution_extra_platform_properties=bootstrap_options.remote_execution_extra_platform_properties,
             remote_execution_headers=bootstrap_options.remote_execution_headers,
-            remote_execution_enable_streaming=bootstrap_options.remote_execution_enable_streaming,
             remote_execution_overall_deadline_secs=bootstrap_options.remote_execution_overall_deadline_secs,
             process_execution_local_enable_nailgun=bootstrap_options.process_execution_local_enable_nailgun,
         )
@@ -138,7 +136,6 @@ def from_bootstrap_options(cls, bootstrap_options):
     remote_oauth_bearer_token_path=None,
     remote_execution_extra_platform_properties=[],
     remote_execution_headers={},
-    remote_execution_enable_streaming=True,
     remote_execution_overall_deadline_secs=60 * 60,  # one hour
     process_execution_local_enable_nailgun=False,
 )
@@ -781,9 +778,12 @@ def register_bootstrap_options(cls, register):
         register(
             "--remote-execution-enable-streaming",
             type=bool,
-            default=DEFAULT_EXECUTION_OPTIONS.remote_execution_enable_streaming,
+            default=True,
             advanced=True,
-            help="Enable the streaming remote execution client.",
+            help="This option no longer does anything. (It used to enable the streaming remote execution client "
+            "which is now the only remote execution client.)",
+            removal_version="2.1.0.dev0",
+            removal_hint="This option is no longer applicable.",
         )
         register(
             "--remote-execution-overall-deadline-secs",