Revert of Adding target to build SwiftShader (patchset #4 id:60001 of...

Revert of Adding target to build SwiftShader (patchset #4 id:60001 of https://codereview.chromium.org/2330173002/ )

Reason for revert:
This seems to not build in our sysroot: https://build.chromium.org/p/chromium.fyi/builders/ClangToTLinuxASan/builds/5244/steps/compile/logs/stdio

Original issue's description:
> Adding target to build SwiftShader
>
> Added the swiftshader target to BUILD.gn's deps in order to enable building SwiftShader using 'ninja -C out/Default swiftshader' or similar command. The 'swiftshader' target already existed in third_party/swiftshader/BUILD.gn, but was unaccessible due to not being referenced in any other BUILD.gn files. Note that SwiftShader does not automatically build with Chromium.
>
> BUG=630728
>
> Committed: https://crrev.com/72ce608cb69854a0029f019e67ddf1509c274f0a
> Cr-Commit-Position: refs/heads/master@{#418934}

TBR=sugoi@google.com,sugoi@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=630728

Review-Url: https://codereview.chromium.org/2349473003
Cr-Commit-Position: refs/heads/master@{#419011}

BUILD.gn

@@ -500,10 +500,6 @@ group("both_gn_and_gyp") {
     ]
   }
 
-  if (is_linux || is_win) {
-    deps += [ "//third_party/swiftshader" ]
-  }
-
   # TODO(GYP): Figure out which of these should (and can) build
   # for chromeos/ios.
   if (!is_chromeos && !is_ios) {