[ios] Rename autofill-prune-suggestions flag

The iOS flag enable-autofill-prune-suggestions is the same
as autofill-prune-suggestions.

Since autofill-prune-suggestions flag name is already in
flag-metadata.json, this CL just changes the name of
the iOS flag to be the same as the one used in other
platforms.

These flags were added in https://crrev.com/c/1565295.

TBR=caitlinfischer@google.com

Bug: 1058614
Change-Id: I395648693b485d08f85f188b99c64a2a695469ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2097123
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749178}

ios/chrome/browser/flags/about_flags.mm

@@ -457,7 +457,7 @@ const flags_ui::FeatureEntry kFeatureEntries[] = {
          autofill::features::kAutofillUseMobileLabelDisambiguation,
          kAutofillUseMobileLabelDisambiguationVariations,
          "AutofillUseMobileLabelDisambiguation")},
-    {"enable-autofill-prune-suggestions",
+    {"autofill-prune-suggestions",
      flag_descriptions::kAutofillPruneSuggestionsName,
      flag_descriptions::kAutofillPruneSuggestionsDescription, flags_ui::kOsIos,
      FEATURE_VALUE_TYPE(autofill::features::kAutofillPruneSuggestions)},