Revert "silence "no common root" warning"

This reverts commit c5ff6a3e.
It did not work (needs to be set as a coq_makefile option) so I moved the flag to the _CoqProject file instead.

buildjob

@@ -51,7 +51,6 @@ fi
 if [[ "$CI_RUNNER_DESCRIPTION" == "coop-timing" ]]; then
     export TIMECMD=ci/perf
 fi
-
 # git-ignore CI-generated files
 mkdir -p .git/info
 cat >> .git/info/exclude <<EOF
@@ -64,7 +63,6 @@ EOF
 
 # Build
 status "[buildjob] Perfoming build (make $MAKE_TARGET)"
-export INSTALLDEFAULTROOT=ci # silence "no common root" warning
 time make --output-sync --no-print-directory -k -j$CPU_CORES $MAKE_TARGET 2>&1 | tee build-log.txt
 
 # Check for warnings