get rid of MBED_OPERATORS

It's impossible to use the operators unless you know they're going to be there. So we should either not define operators (if it's still the case that we need to support a compiler that does not support operator overloading – which seems very unlikely at this point), or always define them.

Search the repository for MBED_OPERATORS and inspect where its operator definitions depend on compiler support. Determine whether any supported compiler still requires the conditional path, then remove the macro or define the operators consistently as appropriate. Run the repository's relevant build and driver tests to confirm operator use remains supported.

Written by the indexing model from the issue text.