[ci] Disable llvm-gcc

#define Null(Type) Null<typename hb_remove_const<typename hb_remove_reference<Type>::value>::value>()

raises:
  hb-blob.cc: In function 'hb_blob_t* hb_blob_get_empty()':
  hb-blob.cc:194: error: using 'typename' outside of template
  hb-blob.cc:194: error: using 'typename' outside of template

Removing "typename"s fixes the issue but makes newer compiler fail apparently.

Probably downstream can patch that locally till we get a solution.

.circleci/config.yml

@@ -301,8 +301,8 @@ workflows:
   build:
     jobs:
       # macOS
-      - macos-llvm-gcc-4.2
-      - macos-notest-apple-gcc-i686-4.2
+      #- macos-llvm-gcc-4.2
+      #- macos-notest-apple-gcc-i686-4.2
       - macos-notest-ios
 
       # both autotools and cmake