Remove -Werror from CMakeLists

-Werror causes an error if an ignorable warning is found. This is
beneficial during development because it ensures that warnings are
fixed, but it is harmful for deployment since it can turn warnings that
may be spurious into blockers for a user to build the project.

Therefore don't specify -Werror in CMakeLists that must be used to build
the project. Instead specify it in outer build scripts that a user will
typically bypass.

The idiomatic way to specify -Werror in CMake 3.24 and later is to use
CMAKE_COMPILE_WARNING_AS_ERROR so use that.