Clean up namespacing and static variable declarations.

[rhornung67]

We have static variable declarations like this in a SYCL file: https://github.com/LLNL/RAJA/blob/develop/include/RAJA/policy/sycl/reduce.hpp#L74

and like this in an OpenMP target file: https://github.com/LLNL/RAJA/blob/develop/include/RAJA/policy/openmp_target/reduce.hpp#L70

The usage of these variables is essentially the same within those two files, but the declarations are inconsistent.

• Make static variable declarations consistent, in this example, it seems that the first code link above would be preferred.
• The namespace scope associated with the variables cited in these code examples is also strange. It seems like sycl::MaxNumTeams should be replaced with RAJA::sycl::MaxNumTeams for clarity. Similarly for the OpenMP target example.