Remove last remains of use_process_wrapper flag. (#1192)

Co-authored-by: UebelAndre <github@uebelandre.com>

rust/private/rustc.bzl

@@ -941,7 +941,7 @@ def rustc_compile_action(
     else:
         # Run without process_wrapper
         if build_env_files or build_flags_files or stamp:
-            fail("build_env_files, build_flags_files, stamp are not supported if use_process_wrapper is False")
+            fail("build_env_files, build_flags_files, stamp are not supported when building without process_wrapper")
         ctx.actions.run(
             executable = toolchain.rustc,
             inputs = compile_inputs,

rust/settings/BUILD.bazel

@@ -29,13 +29,6 @@ bool_flag(
     build_setting_default = False,
 )
 
-# A flag that enables/disables the use of process_wrapper when invoking rustc.
-# This is useful for building process_wrapper itself without causing dependency cycles.
-bool_flag(
-    name = "use_process_wrapper",
-    build_setting_default = True,
-)
-
 bzl_library(
     name = "bzl_lib",
     srcs = glob(["**/*.bzl"]),