[nexus] move TREL configuration to nexus-config header (#12891)

This commit moves the TREL configuration from the build script to the
nexus-config header file. This ensures that TREL is consistently enabled
for all nexus builds and simplifies the build script.

Specifically:
- Added OPENTHREAD_CONFIG_RADIO_LINK_TREL_ENABLE to
  tests/nexus/openthread-core-nexus-config.h.
- Removed OT_TREL from tests/nexus/build.sh and simplified the build
  options.

tests/nexus/build.sh

@@ -46,15 +46,9 @@ fi
 
 case $1 in
     trel)
-        trel=ON
         fifteenfour=OFF
         ;;
-    multi-radio)
-        trel=ON
-        fifteenfour=ON
-        ;;
     *)
-        trel=ON
         fifteenfour=ON
         ;;
 esac
@@ -65,7 +59,7 @@ echo "==========================================================================
 cd "${top_builddir}" || die "cd failed"
 cmake -GNinja -DOT_PLATFORM=nexus -DOT_COMPILE_WARNING_AS_ERROR=ON \
     -DOT_THREAD_VERSION=1.4 -DOT_APP_CLI=OFF -DOT_APP_NCP=OFF -DOT_APP_RCP=OFF \
-    -DOT_15_4=${fifteenfour} -DOT_TREL=${trel} \
+    -DOT_15_4=${fifteenfour} \
     -DOT_PROJECT_CONFIG="${top_srcdir}/tests/nexus/openthread-core-nexus-config.h" \
     "${top_srcdir}" || die
 ninja || die

tests/nexus/openthread-core-nexus-config.h

@@ -151,6 +151,7 @@
 #define OPENTHREAD_CONFIG_TMF_NETDATA_SERVICE_ENABLE 1
 #define OPENTHREAD_CONFIG_TMF_NETDIAG_CLIENT_ENABLE 1
 #define OPENTHREAD_CONFIG_TMF_SNOOP_CACHE_ENTRY_TIMEOUT 3
+#define OPENTHREAD_CONFIG_RADIO_LINK_TREL_ENABLE 1
 #define OPENTHREAD_CONFIG_TREL_MANAGE_DNSSD_ENABLE 1
 #define OPENTHREAD_CONFIG_TREL_USE_HEAP_ENABLE 1
 #define OPENTHREAD_CONFIG_UPTIME_ENABLE 1