[NTP][Realbox] Remove Realbox Theming feature from fieldtrial testing config

Realbox themeing has been flagged by UX not ready for experimentation.
Removing it from Finch (cr/317679487) and fieldtrial testing configs.

Change-Id: Ib047590e8337c820a3847ed836a7cf556bcb7f67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2258073
Auto-Submit: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781560}

testing/variations/fieldtrial_testing_config.json

@@ -4377,7 +4377,6 @@
                     },
                     "enable_features": [
                         "NtpRealbox",
-                        "NtpRealboxMatchOmniboxTheme",
                         "OmniboxDisplayTitleForCurrentUrl",
                         "OmniboxDocumentProvider",
                         "OmniboxHistoryQuickProviderAllowButDoNotScoreMidwordTerms",