Revert "[gn build] Add a file that should have been in cfe02847496b856aa"

This reverts commit 6be7f5c354.
We'll need this file eventually, but it in fact shouldn't have been in
cfe0284749. It's currently unreferenced.

llvm/utils/gn/secondary/lldb/source/Plugins/Architecture/Arm/BUILD.gn

@@ -1,14 +0,0 @@
-static_library("Arm") {
-  output_name = "lldbPluginArchitectureArm"
-  configs += [ "//llvm/utils/gn/build:lldb_code" ]
-  deps = [
-    "//lldb/source/Core",
-    "//lldb/source/Target",
-    "//lldb/source/Utility",
-    "//lldb/source/Plugins/Process/Utility",
-    "//llvm/lib/Support",
-  ]
-  # Uses source-relative paths for own includes.
-  include_dirs = [ "//lldb/source" ]
-  sources = [ "ArchitectureArm.cpp" ]
-}