topological_sorting: replace ineffective continue with break

[conpierce8]

Description of issue

The continue statement in ufl.utils.sorting.topological_sorting (https://github.com/FEniCS/ufl/blob/main/ufl/utils/sorting.py#L25) has no effect: it causes execution to jump to the beginning of the inner for loop, which would happen anyway since there is no code following the continue inside the loop. It should be a break statement, since there is no need to continue searching the edges once node has been removed from S.

Description of changes

This pull request replaces the offending continue with a break.