Remove warnings of CUDA #20012-D.

This MR removes the warnings caused by having default constructors with __host__ __device__ attributes. Default constructors do not need these attributes, and hence CUDA was complaining with warnings similar to these:

warning #20012-D: __host__ annotation is ignored on a function("MultiEventContainer") that is explicitly defaulted on its first declaration

Separately, the fact we did not see this one is due to warnings in CUDA not being treated as errors in the relevant builds, which we should fix (in a separate MR).

added bug fix label

assigned to @msaur

/ci-test --merge

added ci-test-triggered label

• [2021-09-25 13:34] Validation started with lhcb-master-mr#2940

mentioned in issue Moore#324 (closed)

resolved all threads

merged

mentioned in commit f6840638