Do not make directory for source checkout

According to circle docs, if you include the path argument to "checkout":
if the path exists and is a git repo, no clone will be done, only fetch
from origin.  If path exists and is not a git repo, it is an error.

scripts/circle/Dockerfile

@@ -61,7 +61,4 @@ RUN cd source && \
 # Cleanup
 RUN rm -rf /opt/adios1/1.12.0/source
 
-# Make the directory where CircleCI can clone the ADIOS2 source
-RUN mkdir -p /home/adios2/source
-
 WORKDIR /home/adios2