Revert "CI: switch to building against styhead branches where possible"

This reverts commit 2b1348d74f.

Revert to allow the meta-arm master branch to use the master branch of
other layers.

ci/base.yml

@@ -7,7 +7,7 @@ distro: poky
 
 defaults:
   repos:
-    branch: styhead
+    branch: master
 
 repos:
   meta-arm:

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