AGI: Get rid of unused SoundMgr parameters
ENGINES: Silence clang warning about unused private member _vm

This affects the Console / debugger classes of multiple engines.
An alternative solution would have been to remove the unused _vm
member vars. However, it seems likely that in the future, the _vm
member could be useful for methods added to the console. So instead,
we add a simple assert(_vm) to silence the clang warning.
ENGINES: Remove a bunch of unused private member variables

All instances uncovered by clang warnings.
SCI: Remove a bunch of unused private member variables

All instances uncovered by clang warnings.
HOPKINS: Replace non-ASCII characters by octal string literals

This silences a clang warning. It seems like a good idea, too: non-ASCII
chars in string constants are prone to break as a result of (possibly
unintentional) encoding conversions (which one can configure git to
perform).
WINTERMUTE: Do not us char literals to index an array

This silences a clang warning
VIDEO: Remove unused CDToonsDecoder::_currentFrame member var
AUDIO: Remove unused MixerImpl::_syst member var
AGI: Silence another clang var

Reading an array without using the resulting value has no effect.
In any case, this code still looks like it may be broken, so somebody
who knows more about AGI should perhaps investigate this closer.