Revert "Disable -Wdeprecated-copy with tip-of-tree Clang"

This reverts commit 0ff2da3a.

Reason for revert:
The new warning was reverted upstream (see bug).

Original change's description:
> Disable -Wdeprecated-copy with tip-of-tree Clang
> 
> while we figure out how to handle this new warning.
> 
> TBR=rnk
> 
> Bug: 1028110
> Change-Id: I88be5c763bd0f2fcb40ebecc20990f96853d70c2
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1932804
> Reviewed-by: Hans Wennborg <hans@chromium.org>
> Commit-Queue: Hans Wennborg <hans@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#718651}

TBR=hans@chromium.org,rnk@chromium.org

Change-Id: I0e0babea932ff9ae3fd92530cc96e1c6547429a6
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1028110
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1935111Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719051}

build/config/compiler/BUILD.gn

@@ -1549,13 +1549,6 @@ config("default_warnings") {
           # TODO(https://crbug.com/995993): Clean up and enable.
           "-Wno-implicit-fallthrough",
         ]
-
-        if (llvm_force_head_revision) {
-          cflags += [
-            # TODO(https://crbug.com/1028110): Evaluate and possible enable.
-            "-Wno-deprecated-copy",
-          ]
-        }
       }
     }
   }