Remove unused IsValidHostname utility method.
Remove unused IsValidHostname utility method.

Usages were removed in revision 62c75d9af9 ("Removed all informs code
(no more 1.3.x UI)").

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