Remove obsolete __clang__ check (and -D).

We've been all-clang, all the time for years now.

Change-Id: I1296a762335e614529ffd92c2a6b26f87861361b

Author: enh-google

tests/Android.bp

@@ -719,11 +719,6 @@ cc_defaults {
     ],
     srcs: ["clang_fortify_tests.cpp"],
     tidy: false,
-    target: {
-        host: {
-            cflags: ["-D__clang__"],
-        },
-    },
 }
 
 cc_test_library {

tests/clang_fortify_tests.cpp

@@ -14,10 +14,6 @@
  * limitations under the License.
  */
 
-#ifndef __clang__
-#error "Non-clang isn't supported"
-#endif
-
 //
 // Clang compile-time and run-time tests for Bionic's FORTIFY.
 //