Cocoa: Fix install name for installed dylib

The install name was incorrectly set to a relative path.

This change leaves the install name of the installed dylib as
@rpath/soname.  Those who wish to override this can set the
CMAKE_INSTALL_NAME_DIR variable.

Closes #1504.

src/CMakeLists.txt

@@ -203,9 +203,6 @@ if (BUILD_SHARED_LIBS)
         endif()
 
         target_compile_definitions(glfw INTERFACE GLFW_DLL)
-    elseif (APPLE)
-        set_target_properties(glfw PROPERTIES
-                              INSTALL_NAME_DIR "${CMAKE_INSTALL_LIBDIR}")
     endif()
 
     if (MINGW)