remove_self_loops mutates dense/sparse GNNGraph inputs in place

`remove_self_loops` is also changing the original graph which is not the expected behaviour

MWE

```julia
using GNNGraphs

g = GNNGraph([1 1; 0 1]; graph_type = :dense)
g2 = remove_self_loops(g)

adjacency_matrix(g)   # g will be changed
adjacency_matrix(g2)  # same
g.graph === g2.graph
```