Remove deprecated flags from flag-metadata.json

The enable-webrtc-hw-{h264,vp8,vp9}-encoding flags were previously
removed. This change also removes them from flag-metadata.json.

These flags were removed in this CL:
http://crrev.com/c/2033246

Bug: 599650
Change-Id: I83e31396d8a2506d3ed6a7cc548a9b9cd53e1c4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2050828Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: Rahul Singh <rahsin@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#740564}

chrome/browser/flag-metadata.json

@@ -2123,21 +2123,6 @@
     "owners": [ "qingsi" ],
     "expiry_milestone": 85
   },
-  {
-    "name": "enable-webrtc-hw-h264-encoding",
-    "owners": [ "hiroh", "chromeos-video-eng@google.com" ],
-    "expiry_milestone": 83
-  },
-  {
-    "name": "enable-webrtc-hw-vp8-encoding",
-    "owners": [ "hiroh", "chromeos-video-eng@google.com" ],
-    "expiry_milestone": 83
-  },
-  {
-    "name": "enable-webrtc-hw-vp9-encoding",
-    "owners": [ "hiroh", "chromeos-video-eng@google.com" ],
-    "expiry_milestone": 83
-  },
   {
     "name": "enable-webrtc-hybrid-agc",
     "owners": [ "aleloi" ],