removed all constraints from custom toolchain

third_party/toolchains/BUILD

@@ -79,19 +79,8 @@ platform(
 toolchain(
     name = "cc-toolchain-clang-x86_64-default",
     exec_compatible_with = [
-        "@bazel_tools//platforms:linux",
-        "@bazel_tools//platforms:x86_64",
-        "@bazel_tools//tools/cpp:clang",
-        "@com_github_bazelbuild_bazeltoolchains//constraints:xenial",
-        "@com_github_bazelbuild_bazeltoolchains//constraints/sanitizers:support_msan",
-        "//third_party/toolchains/machine_size:standard",
-        "//third_party/toolchains/machine_size:large",
     ],
     target_compatible_with = [
-        "//third_party/toolchains:rbe_ubuntu1604",
-        "//third_party/toolchains:rbe_ubuntu1604_large",
-        "@bazel_tools//platforms:linux",
-        "@bazel_tools//platforms:x86_64",
     ],
     toolchain = "@com_github_bazelbuild_bazeltoolchains//configs/ubuntu16_04_clang/1.1/bazel_0.16.1/default:cc-compiler-k8",
     toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",