CI: remove branch name

The meta-arm styhead branch should be using the styhead branches of
meta-clang, meta-virtualization, and meta-secure-core.  Remove the
master branch specific line to allow for these to be used.

Signed-off-by: Jon Mason <jon.mason@arm.com>

ci/clang.yml

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

ci/meta-secure-core.yml

@@ -6,7 +6,6 @@ header:
 repos:
   meta-secure-core:
     url: https://github.com/Wind-River/meta-secure-core.git
-    branch: master
     layers:
       meta-secure-core-common:
       meta-signing-key:

ci/meta-virtualization.yml

@@ -8,4 +8,3 @@ header:
 repos:
   meta-virtualization:
     url: https://git.yoctoproject.org/meta-virtualization
-    branch: master