.github/workflows/build.yml: do not install yajl on macOS

The Brew package of yajl is broken, it fails to build:

 In file included from ../src/lib/yajl/Handle.cxx:4:
 In file included from ../src/lib/yajl/Handle.hxx:6:
 /opt/homebrew/Cellar/yajl/2.1.0/include/yajl/yajl_parse.h:22:10: fatal error: 'yajl/yajl_common.h' file not found
 #include <yajl/yajl_common.h>
          ^~~~~~~~~~~~~~~~~~~~
 1 error generated.

Therefore, MPD cannot support building with yajl on macOS.

Author: MaxKellermann

.github/workflows/build.yml

@@ -156,7 +156,6 @@ jobs:
             icu4c \
             ffmpeg \
             libnfs \
-            yajl \
             libupnp \
             libid3tag \
             chromaprint \