Remove CORE_UTIL_EXPORT

[ Merge of http://go/wvgerrit/155070 ]

These functions are internal utilities and should not be exported from
our library.  IIRC, this was added to aid in Windows tests, but this is
no longer needed.

Bug: 236317198
Test: build_and_run_all_unit_tests.sh
Change-Id: I19beb35a15f0f87dc8f968e1dd5302aed9463e54

libwvdrmengine/cdm/util/include/rw_lock.h

@@ -16,7 +16,7 @@
 namespace wvutil {
 
 // A simple reader-writer mutex implementation that mimics the one from C++17
-class CORE_UTIL_EXPORT shared_mutex {
+class shared_mutex {
  public:
   shared_mutex() : reader_count_(0), has_writer_(false) {}
   ~shared_mutex();