Remove "v8_use_snapshot" from BUILD.gn

nosnapshot builds were deprecated in V8 v7.4 and are now removed.
To be able to remove "v8_use_snapshot" upstream, we first need to
remove the single use-site in Chromium.

Corresponding V8 CL: https://crrev.com/c/1784282.

Bug: v8:8531
Change-Id: Id0f393220f88e77b78b42e2a6314b8dea5d96b6c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1796325Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705855}

BUILD.gn

@@ -932,7 +932,7 @@ group("gn_visibility") {
 
   if (!is_ios) {
     deps += [ "//v8:postmortem-metadata" ]
-    if (v8_use_snapshot && !v8_use_external_startup_data) {
+    if (!v8_use_external_startup_data) {
       deps += [ "//v8:v8_snapshot" ]
     }
   }