Derive it from `$BUNDLE` instead. autotools seems to be already relying
on `$BUNDLE_SRC` being equal to `$BUNDLE.c`. (I haven't realized this
before aaebb45f58.)
Also drop redundant `nodist_<target>_SOURCE` lines in tunits and units.
Follow-up to aaebb45f58#17688
Follow-up to 2c27a67daa#17590Closes#17692