[CMake/Make/bin] Remove vestigial MPI support

[SFrijters]

Description

PR #608 removed MPI support, after this compile_cpp_mpi_example_with_modes was
unused so it can be removed as well.

[codecov]

Codecov Report

Merging #611 (65409a1) into development (d30000b) will increase coverage by 0.02%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           development     #611      +/-   ##
===============================================
+ Coverage        77.25%   77.28%   +0.02%     
===============================================
  Files              262      258       -4     
  Lines            19568    19394     -174     
===============================================
- Hits             15118    14988     -130     
+ Misses            4450     4406      -44     

Impacted Files	Coverage Δ
src/occa/internal/bin/occa.cpp	37.32% <ø> (+0.12%)	⬆️
src/c/kernel_fortran_interface.cpp	0.00% <0.00%> (-10.00%)	⬇️
src/occa/internal/utils/store.hpp	90.62% <0.00%> (-6.25%)	⬇️
src/occa/internal/utils/string.hpp	92.98% <0.00%> (-4.72%)	⬇️
src/occa/internal/utils/sys.cpp	73.95% <0.00%> (-3.31%)	⬇️
src/dtype/builtins.cpp	44.44% <0.00%> (-2.78%)	⬇️
src/c/base.cpp	84.88% <0.00%> (-2.33%)	⬇️
src/occa/internal/modes/serial/registration.cpp	91.11% <0.00%> (-2.23%)	⬇️
src/occa/internal/utils/env.cpp	81.01% <0.00%> (-1.27%)	⬇️
...a/internal/lang/statement/declarationStatement.cpp	94.00% <0.00%> (-1.05%)	⬇️
... and 11 more

[kris-rowe]

Good catch!