Refactor ICE_VERSION logic in the OMERO homebrew installation script (rebased onto develop)



This is the same as gh-1207 but rebased onto develop.

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- Rely upon `brew deps omero $BREW_OPTS` to detect installed Ice version
- Remove `OMERO_ALT` variable (replaced by the scc merge logic in the library)

To be tested together with ome/homebrew-alt#31 using `BREW_OPTS=--with-ice34`