CI: use kirkstone branches for meta-clang and meta-virtualization

These branches have kirkstone branches now, so use them.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

ci/clang.yml

@@ -4,7 +4,6 @@ header:
 repos:
   meta-clang:
     url: https://github.com/kraj/meta-clang
-    refspec: master
 
 local_conf_header:
   clang: |

ci/meta-virtualization.yml

@@ -6,4 +6,3 @@ header:
 repos:
   meta-virtualization:
     url: git://git.yoctoproject.org/meta-virtualization
-    refspec: master