mixer-control: Don't specify default c_marshaller

Apart from being less code, this actually gives us a tiny performance
improvement. Up until a few years ago, if you pass `NULL` as the
marshaller for a signal, GLib would fall back to
`g_cclosure_marshal_generic` which uses libffi to pack/unpack its
arguments. One could avoid this by specifying a more specific
marshaller which would then be used to immediately pack and unpack into
GValues with the correct type.

Lately however, as a way of optimizing signal emission (which can be
quite expensive), GLib added a possibility to set a `va_marshaller`,
which skips the unnecessary GValue packing and unpacking and just uses a
valist variant.

Since the performance difference is big enough, if the marshaller
argument is NULL, `g_signal_new()` will now check for the simple
marshallers (return type NONE and a single argument) and set both the
generic and the valist marshaller. In other words, less code for us with
behind-the-scenes optimizations.

In case you also want va_marshallers for more complex signals, you can
use `g_signal_set_va_marshaller()`.

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