Refactor ICE_VERSION logic in the OMERO homebrew installation script

- 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)
Remove Homebrew/pip cleanup from the installation script

This cleanup step will be handled in the job directly most certainly using
`rm -rf /usr/local/*` to decrease the maintenance burden.
The TESTING_MODE variable only stipulates whether homebrew-alt PRs should be
merged for now.
Merge pull request #1215 from sbesson/rebased/develop/homebrew_ice34

Refactor ICE_VERSION logic in the OMERO homebrew installation script (rebased onto develop)
Merge pull request #1216 from sbesson/homebrew_cleanup

Remove Homebrew/pip cleanup from the installation script