Remove unused Strings.Common members.
Remove unused Strings.Common members.

The last references were removed in 5170c6ad1d ("Remove unused
CommonStrings class"), but the last real usages were removed in revision
62c75d9af9 ("Removed all informs code (no more 1.3.x UI)").

mnaiman/duplicati,duplicati/duplicati,mnaiman/duplicati,duplicati/duplicati,mnaiman/duplicati,duplicati/duplicati,mnaiman/duplicati,mnaiman/duplicati,duplicati/duplicati,duplicati/duplicati