[Flang][OpenMP] Remove unused OpWithBodyGenInfo attributes (#97572)

This patch removes the `outerCombined`, `reductionSymbols` and
`reductionTypes` attributes from the `OpWithBodyGenInfo` structure and
their uses, as they never impact the lowering process or its output.

The `outerCombined` variable is always set to `false`, so in practice it
doesn't represent what its name indicates. Furthermore, initializing it
correctly can result in privatization not being performed in cases where
it should (at least tests doing this together with composite construct
support pointed me in that direction). It seems to be tied to the early
privatization approach, where a redundant alloca could possibly be
avoided in certain cases. With the transition to delayed privatization,
it seems like it won't serve that purpose anymore, since the decision of
what and where privatization-related allocations are inserted will be
postponed to the MLIR to LLVM IR translation stage. Since this feature
is already currently not being used, its potential benefit appears to be
minor and it won't make sense to do once the delayed privatization
approach is rolled out, I propose removing it.

The `reductionSymbols` and `reductionTypes` variables are set in certain
cases but never used. Unless there's a plan where these will be needed,
in which case it would be a better alternative to document it, I believe
we should also remove them.