Bug 1695836 - Remove leftover from bug 1694775. r=firefox-build-system-reviewers,andi,dmajor

As of that bug, the default docker image for toolchain builds is
deb10-toolchain-build, so setting the docker image in the task is
redundant.

Differential Revision: https://phabricator.services.mozilla.com/D106874

taskcluster/ci/toolchain/misc.yml

@@ -74,8 +74,6 @@ linux64-libdmg:
     description: "libdmg-hfsplus toolchain build"
     treeherder:
         symbol: TL(libdmg-hfs+)
-    worker:
-        docker-image: {in-tree: deb10-toolchain-build}
     run:
         script: build-libdmg-hfsplus.sh
         toolchain-artifact: public/build/dmg.tar.xz