Stop defining V8_DEPRECATION_WARNINGS.

https://chromium-review.googlesource.com/c/v8/v8/+/1151110/ made it so
that V8_DEPRECATION_WARNINGS is defined for all targets depending on v8,
which is all the targets that need this define really. So stop setting
it manually.

Change-Id: Ib0044f5a160a186e45873dae0286541f14fd3fbc
Reviewed-on: https://pdfium-review.googlesource.com/c/48010
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 3a2cf0a..52c22d6 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -77,7 +77,7 @@
     ":pdfium_common_config",
     ":pdfium_public_config",
   ]
-  defines = [ "V8_DEPRECATION_WARNINGS" ]
+  defines = []
   if (is_clang) {
     cflags += [ "-Wshadow" ]
   }