Initial port of PathAliasDelete with test. Currently works for deleting ...

...a single url alias.

The original D7 function was the create or delete URL alias and thus had two inputs - source and alias. If the source was missing but an alias provided it would delete the alias, and if the alias was missing and a source provided then it would delete aliases associated with that source. 

Now that the create and delete functions are split, the delete only needs one input, i.e. what you want to delete. This initial patch provides the code to delete a single url alias as the class name suggests. 

I believe further discussion is needed to decide how this functionality is going to work in 8, i.e. can we still pass it a source and delete all relevant aliases (how do we check whether it's source or alias?) - can we send it an array of sources and/or aliases, etc. 

So, for the moment, this is a start.
I think this is wrong - AliasStorageInterface tells me that the delete() method expects a $conditions array?