BibMerge: delete duplicate record first

• When merging and deleting records, do first submit the deleted one such that any possible identifier that would result in a duplicate are removed first, before updating the remaining record. (closes #1645)

Signed-off-by: Jerome Caffaro <jerome.caffaro@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>