Delete obsolete use-pdf-compositor-service-for-print flag entry.

The flag was deleted in https://crrev.com/708103, so the corresponding
entry in flag-metadata.json is useless.

Bug: 824867
Change-Id: Ice9bab31555bd0012ab349f636937a8676da2e0d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1902748
Auto-Submit: Lei Zhang <thestig@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713430}

chrome/browser/flag-metadata.json

@@ -3539,11 +3539,6 @@
     "owners": [ "claudiomagni" ],
     "expiry_milestone": 76
   },
-  {
-    "name": "use-pdf-compositor-service-for-print",
-    "owners": [ "//printing/OWNERS" ],
-    "expiry_milestone": 79
-  },
   {
     "name": "use-search-click-for-right-click",
     "owners": [ "zentaro" ],