build: Disallow explicitly setting concurrent_links in lto builds.

No bots should set this anymore, so we can make the assert a bit stricter.

Bug: 1132930
Change-Id: Ic76c777faf4b3d58761c3063b6371bc1a7e18393
Cq-Include-Trybots: chromium/try:chromeos-amd64-generic-cfi-thin-lto-rel
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2472267
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821538}

build/toolchain/concurrent_links.gni

@@ -92,8 +92,7 @@ if (concurrent_links == -1) {
     java_cmd_pool_size = _command_dict.secondary_pool_size
   }
 } else {
-  assert(!use_thin_lto || concurrent_links == 1,
-         "can't explicitly set concurrent_links with thinlto")
+  assert(!use_thin_lto, "can't explicitly set concurrent_links with thinlto")
   concurrent_links_logs =
       [ "concurrent_links set by GN arg (value=$concurrent_links)" ]
 }