Revert "[DNR] Change listed availability of Modifyheaderinfo"

This reverts commit a4dc12c8.

Reason for revert: 

This breaks an assertion in the Docserver causing it to stop 
updating. 

Original change's description:
> [DNR] Change listed availability of Modifyheaderinfo
> 
> The structs were already functional by M85, but because the
> documentation was updated after the branch cut, the docserver listed
> ModifyheaderInfo as being available at M86.
> 
> The availability is usually determined automatically by looking over API
> schemas over different versions/channels in
> chrome/common/extensions/docs/server2/availability_finder.py
> 
> Bug: 947591
> Change-Id: Iae71ac3c7b6bc9fb7e7d86815e855e31f588f36e
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2340074
> Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
> Commit-Queue: Kelvin Jiang <kelvinjiang@chromium.org>
> Auto-Submit: Kelvin Jiang <kelvinjiang@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#795805}

TBR=karandeepb@chromium.org,kelvinjiang@chromium.org

Change-Id: Ic9c8fdfed838d8e5e134db029e4220ef1f677c90
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 947591, 1114299
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343946Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796185}

chrome/common/extensions/docs/templates/json/api_availabilities.json

@@ -3,10 +3,6 @@
 // The versions listed here were drawn either from hardcoded HTML intro tables
 // or from the whats_new page.
 {
-  "declarativeNetRequest.ModifyHeaderInfo": {
-    "channel": "stable",
-    "version": 85
-  },
   "devtools.inspectedWindow": {
     "channel": "stable",
     "version": 18