sync: remove source or source_url if removed in prob-specs

[ee7]

If a source or source_url key/value pair was removed from an
exercise's metadata.toml file in exercism/problem-specifications,
configlet sync -u would incorrectly set an existing corresponding
.meta/config.json value to the empty string.

This would cause configlet lint to indicate an error, because
a source value must be a non-blank string, and a source_url value
must be a URL.

Instead, make configlet sync remove any source or source_url
key/value pair that was removed upstream.

Refs: exercism/problem-specifications@05999f34ab02
Fixes: #608

---

I'll check this PR again later, but I think it's close:

$ cd /tmp
$ git clone https://github.com/exercism/elm
$ cd elm
$ git checkout 8149a96a5c2c
$ configlet sync --metadata -e grade-school -uy
Updating cached 'problem-specifications' data...
Checking exercises...
[warn] metadata: unsynced: grade-school
Updated the metadata for 1 Practice Exercise
The `grade-school` exercise has up-to-date metadata!
$ git diff

--- a/exercises/practice/grade-school/.meta/config.json
+++ b/exercises/practice/grade-school/.meta/config.json
@@ -22,6 +22,5 @@
     ]
   },
   "blurb": "Given students' names along with the grade that they are in, create a roster for the school.",
-  "source": "A pairing session with Phil Battos at gSchool",
-  "source_url": "http://gschool.it"
+  "source": "A pairing session with Phil Battos at gSchool"
 }

[ErikSchierboom]

Nice