Remove unnecessary third_party links: nss, llvm-build, syzygy

third_party/nss is no longer used.
third_party/llvm-build and third_party/syzygy links are not be needed;
these dirs are created during 'gclient runhooks' now that we moved
to executing hooks in our own DEPS file instead of the ones in
chromium/src/DEPS as per https://codereview.webrtc.org/2524673002

BUG=webrtc:5006
NOTRY=True
TESTED=Ran all compile trybots with the --clobber flag.

Review-Url: https://codereview.webrtc.org/2570993003
Cr-Commit-Position: refs/heads/master@{#15621}

.gitignore

@@ -129,7 +129,6 @@
 /third_party/lss
 /third_party/mockito
 /third_party/modp_b64
-/third_party/nss
 /third_party/oauth2
 /third_party/objenesis
 /third_party/ocmock

setup_links.py

@@ -49,9 +49,7 @@ DIRECTORIES = [
   'third_party/libsrtp',
   'third_party/libvpx',
   'third_party/libyuv',
-  'third_party/llvm-build',
   'third_party/lss',
-  'third_party/nss',
   'third_party/ocmock',
   'third_party/openh264',
   'third_party/openmax_dl',
@@ -59,7 +57,6 @@ DIRECTORIES = [
   'third_party/proguard',
   'third_party/protobuf',
   'third_party/sqlite',
-  'third_party/syzygy',
   'third_party/usrsctp',
   'third_party/yasm',
   'third_party/zlib',