rMOZILLARELEASE89ecd6ee8edd

Bug 1492362: [taskgraph] Don't delete fetches after running a task; r=glandium

In Bug 1466660, we started deleting the fetches after a task had run, to avoid
interference between tasks. It turns out the only tasks this was for were the
`source-test-jsshell` tasks, which were changed to use an absolute directory in
Bug 1465181. However, since Bug 1568460 we've always used a per-task directory
for fetches, so can remove the work-around of removing fethes.

Differential Revision: https://phabricator.services.