Don't use (DY)LD_LIBRARY_PATH as plugin search paths (#4245)

Only use the OIIO_LIBRARY_PATH environment variable by default.

Searching through such system paths has some risk loading plugins
installed for another purpose or that may be incompatible.

If the previous behavior is desired, applications can add these
environment variables to the plugin search path manually.

Signed-off-by: Brecht Van Lommel <brecht@blender.org>

src/libOpenImageIO/imageioplugin.cpp

@@ -469,12 +469,6 @@ pvt::catalog_all_plugins(std::string searchpath)
 
     std::unique_lock<std::recursive_mutex> lock(imageio_mutex);
     append_if_env_exists(searchpath, "OIIO_LIBRARY_PATH", true);
-#ifdef __APPLE__
-    append_if_env_exists(searchpath, "DYLD_LIBRARY_PATH");
-#endif
-#if defined(__linux__) || defined(__FreeBSD__)
-    append_if_env_exists(searchpath, "LD_LIBRARY_PATH");
-#endif
 
     size_t patlen = pattern.length();
     std::vector<std::string> dirs;