jobs_solve_fetch(): remove unused `flag' variable.

scan-build (clang's static analyzer) did complain about `flag` being unused. This patch just removes it but it may be a bug where `flag` should be used but is not.

see 8a3385f5a8997003bff9305c91fe783de9b13e86 where `flag` is unused and removed from `jobs_solve_upgrade` and unused from `jobs_solve_fetch`  but not removed. Hence I believe that this PR, which remove `flag` from `jobs_solve_fetch` seems reasonable.