Delete unused multithreading macro

[jbreue16]

The macro CADET_PARFOR_END extends to a closed bracket when the project is build with multithreading enabled. This causes issues in some environments (build failed on MacOS), when the corresponding opening bracket is not defined.
This commit deletes unused CADET_PARFOR_END statements.

fixes #353

[spdavern]

This resolved the errors with LumpedRateModelWithPoresDG2D.cpp. Upon attempting building with these changes I encountered similar issues with LumpedRateModelWithPoresDG2D-InitialConditions.cpp lines 848 and 1290. Upon removing the same CADET_PARFOR_END; code on those two lines I was able to successfully build CADET-Core with the -DENABLE_THREADING=ON cmake flag. Also, the resulting CADET-Core allowed me to set nthreads to 5 and solve a simulation.

I tried to push a commit with the changes - as minimal as they are, but don't have permission. :-)

Thank you for such a quick and effective resolution!

[jbreue16]

Thanks for reporting! :)

[jbreue16]

Instead of removing the unused macros, Ive added parallel support of the corresponding loops as per the last commit.

@spdavern would you be so kind and build this branch again with the new changes - Ive got no MacOS machine at hand rn.
Ill merge the changes into master afterwards.

[spdavern]

Hey @jbreue16!
Here is the result of my attempt to:

cd Cadet
git checkout fix/parallel_build_macOS
git pull      # Verified the changes you submitted: Commit 43b78c28 checked out
cd build
make clean
cd ..
rm -rf build
rm -rf install
mkdir build
mkdir install
cd build
cmake -DCMAKE_INSTALL_PREFIX="<ROOT>/CADET/install" -DENABLE_THREADING=ON ../     # <Root> replaced.

Not sure all those steps are exactly required, but for full disclosure.
I got these errors:

[jbreue16]

The updated commit works, thanks again for your help @spdavern