Drop TESTONLY from sample target. (#6485)

`static_library_demo` is not a test, so it can't depend on a `TESTONLY` target. We don't check this in CMake, outside of `-DIREE_BUILD_TESTS=OFF` builds, which will fail at configure time.

iree/samples/static_library/CMakeLists.txt

diff --git a/iree/samples/static_library/CMakeLists.txt b/iree/samples/static_library/CMakeLists.txt
index 4c818b8..a2f66e8 100644
--- a/iree/samples/static_library/CMakeLists.txt
+++ b/iree/samples/static_library/CMakeLists.txt
@@ -63,7 +63,6 @@
     simple_mul_c.h
   FLATTEN
   PUBLIC
-  TESTONLY
 )
 
 iree_cc_binary(