Remove the remaining host_fragments= arguments. (#2472)

diff --git a/proto/prost/private/prost.bzl b/proto/prost/private/prost.bzl
index 0a9f350..d3c34c7 100644
--- a/proto/prost/private/prost.bzl
+++ b/proto/prost/private/prost.bzl
@@ -275,7 +275,6 @@
         ),
     } | RUSTC_ATTRS,
     fragments = ["cpp"],
-    host_fragments = ["cpp"],
     toolchains = [
         TOOLCHAIN_TYPE,
         "@bazel_tools//tools/cpp:toolchain_type",
diff --git a/proto/protobuf/proto.bzl b/proto/protobuf/proto.bzl
index 7b25430..a9509ee 100644
--- a/proto/protobuf/proto.bzl
+++ b/proto/protobuf/proto.bzl
@@ -325,7 +325,6 @@
         ),
     },
     fragments = ["cpp"],
-    host_fragments = ["cpp"],
     toolchains = [
         str(Label("//proto/protobuf:toolchain_type")),
         str(Label("//rust:toolchain_type")),
@@ -418,7 +417,6 @@
         ),
     },
     fragments = ["cpp"],
-    host_fragments = ["cpp"],
     toolchains = [
         str(Label("//proto/protobuf:toolchain_type")),
         str(Label("//rust:toolchain_type")),
diff --git a/rust/private/unpretty.bzl b/rust/private/unpretty.bzl
index 81b04e0..024a2fc 100644
--- a/rust/private/unpretty.bzl
+++ b/rust/private/unpretty.bzl
@@ -226,7 +226,6 @@
 rust_unpretty_aspect = aspect(
     implementation = _rust_unpretty_aspect_impl,
     fragments = ["cpp"],
-    host_fragments = ["cpp"],
     attrs = {
         "_unpretty_modes": attr.label(
             doc = "The values to pass to `--unpretty`",