Remove dependency on no longer existing target //base/util/timer:timer

This was initially needed as per some changes that were going to be
added as part of PR #8124[1] (in commit 8ba9ddb[2]), that used
util::WallClockTimer, but the merged PR ended up implementing things
differently and this dependency was left around in the BUILD.gn file.

Since //base/util/timer does not exist anymore in Chromium 94 (i.e it
has been moved to //base/timer), this is breaking the build now so
let's remove the stray dependency.

[1] #8124
[2] 8ba9ddb5

components/brave_shields/browser/BUILD.gn

@@ -48,7 +48,6 @@ static_library("browser") {
 
   deps = [
     "//base",
-    "//base/util/timer:timer",
     "//brave/common:pref_names",
     "//brave/components/adblock_rust_ffi",
     "//brave/components/brave_component_updater/browser",