Remove obsolete problems key

The `problems` key has been obsoleted as per [this issue](exercism/DEPRECATED.x-api#137). This has already been succesfully implemented by the [C#](https://github.com/exercism/xcsharp#130), [F#](https://github.com/exercism/xfsharp#224) and [Haskell](exercism/haskell#399) tracks, so API-wise this is a safe action.

Removing the `problems` key also makes it very impossible for people to only add a new exercise in the old, obsolete `problems` key.

config.json

@@ -3,39 +3,6 @@
   "language": "PHP",
   "repository": "https://github.com/exercism/xphp",
   "active": true,
-  "problems": [
-    "hello-world",
-    "hamming",
-    "gigasecond",
-    "bob",
-    "pangram",
-    "rna-transcription",
-    "raindrops",
-    "isogram",
-    "difference-of-squares",
-    "largest-series-product",
-    "roman-numerals",
-    "pig-latin",
-    "robot-name",
-    "leap",
-    "word-count",
-    "anagram",
-    "trinary",
-    "bowling",
-    "clock",
-    "wordy",
-    "connect",
-    "minesweeper",
-    "change",
-    "phone-number",
-    "beer-song",
-    "atbash-cipher",
-    "binary",
-    "braket-push",
-    "accumulate",
-    "variable-length-quantity",
-    "acronym"
-  ],
   "deprecated": [
 
   ],