Undefining `__BLOCKS__` for preprocessing

[sim642]

In https://github.com/goblint/analyzer/pull/597#discussion_r810470622, it came up again that we have this weird __BLOCKS__ undefine: https://github.com/goblint/analyzer/blob/b11a60cd5896b41f9e0c4847825d21f045dc6cc1/src/maingoblint.ml#L161-L163

This is something MacOS specific. So the question is whether it's still required for some reason (and thus should be better commented and consistently applied to compilation database and Makefile analysis as well) or it's a relic that could be removed (maybe it was only needed when using MacOS gcc instead of the generic one we ask for now)?