Revert "Fix mips and mips64 builds."

This reverts commit b5b70be878.

If __unix__ is defined on mips/mips64 (other CL under the same topic),
we will avoid warnings such as:

  * external/libcxx/src/thread.cpp:105:9: error: hardware_concurrency
    not yet implemented

Test: successful build and boot aosp_mips-eng
Test: successful build and boot aosp_mips64-eng

Android.bp

@@ -52,14 +52,6 @@ cc_library_static {
     local_include_dirs: ["include"],
     export_include_dirs: ["include"],
     cflags: ["-Wall", "-Werror"],
-    arch: {
-        mips: {
-            cflags: ["-Wno-#warnings"],
-        },
-        mips64: {
-            cflags: ["-Wno-#warnings"],
-        },
-    },
     cppflags: [
         "-std=c++14",
         "-nostdinc++",