Use STABLE_DMD only for dpl-docs

[wilzbach]

Follow-up to #1926.
If we ever need rdmd, we can trivially built it from tools, which is already available.
Next step would be to replace the STABLE_DMD download with only a download of the DUB binary.

[dlang-bot]

Thanks for your pull request, @wilzbach!

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[PetarKirov]

../dmd/generated/linux/release/64/dmd -run modlist.d ../druntime ../phobos ../dmd  \
	--dump object --dump std --dump etc --dump core --dump "${DMD_SRC_NAME}" >.generated/modlist-release.ddoc
.generated/stable_dmd-2.072.2/dmd2/linux/bin64/dub build --single --compiler=../dmd/generated/linux/release/64/dmd assert_writeln_magic.d
Error: cannot find source code for runtime library file 'object.d'
       dmd might not be correctly installed. Run 'dmd -man' for installation instructions.
       config file: /home/circleci/dmd/generated/linux/release/64/dmd.conf
import path[0] = ../dmd/generated/linux/release/64/../../../../../druntime/import
import path[1] = ../dmd/generated/linux/release/64/../../../../../phobos
posix.mak:421: recipe for target '.generated/modlist-release.ddoc' failed

Do we really have druntime and phobos placed on those locations?