gvc: remove unused 'gvevent_modify'

gvc: remove unused 'gvevent_modify'

The `modify` field was added to `gvdevice_callbacks_t` in
50df9e82edcb61c5650f81ca317a31c9493ed07e, but seemingly has never been used by
any in-tree plugin. This commit removes a no-op, never-called callback for it,
squashing some -Wunused-parameter warnings.

This commit does not remove the member because it is part of the public API. We
are assuming the convention is to check these pointers against null before
calling them, like the other GVC structs. But the struct has no comment
explaining it nor how it is meant to be used, so we cannot be sure how/if users
would be relying on this.