arc: Delete smart-text-selection flag

Previous CLs got rid of this flag in code as it was expired, this CL
only removes said flag from the flag-metadata file that keeps track of
flags and their milestones.

Bug: None
Test: build.
Change-Id: I476bf49ad50b189d05c919883052202f80327b56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2515180
Commit-Queue: David Jacobo <djacobo@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Auto-Submit: David Jacobo <djacobo@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823391}

chrome/browser/flag-metadata.json

@@ -4326,11 +4326,6 @@
     "owners": [ "vuhung", "shaktisahu"],
     "expiry_milestone": 89
   },
-  {
-    "name": "smart-text-selection",
-    "owners": [ "djacobo", "linben" ],
-    "expiry_milestone": 76
-  },
   {
     "name": "smbfs-file-shares",
     "owners": [ "simmonsjosh@google.com", "dats" ],