build: Remove leftover -enable-dse-memoryssa=false flag

These flags were removed in crrev.com/824947 but the thinlto flag was
accidentally left behind.

Bug: 1127713
Change-Id: Iea53c31a0f4a0a7ea09153595646ca6d13a9d45d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2627316
Commit-Queue: Hans Wennborg <hans@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Hans Wennborg <hans@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843026}

build/config/compiler/BUILD.gn

@@ -627,9 +627,6 @@ config("compiler") {
         # Experimentally determined to yield a reasonable trade-off between
         # build time, run-time performance, and binary size.
         "-mllvm:-import-instr-limit=10",
-
-        # TODO(https://crbug.com/1127713): Investigate, remove.
-        "-mllvm:-enable-dse-memoryssa=false",
       ]
     } else {
       ldflags += [ "-flto=thin" ]