ci: Drop -DENABLE_EXTERNAL_SIGNER=ON configure option

This option is now enabled by default.

ci/test/03_test_script.sh

@@ -118,7 +118,7 @@ BASE_BUILD_DIR=${BASE_BUILD_DIR:-$BASE_SCRATCH_DIR/build-$HOST}
 mkdir -p "${BASE_BUILD_DIR}"
 cd "${BASE_BUILD_DIR}"
 
-BITCOIN_CONFIG_ALL="$BITCOIN_CONFIG_ALL -DENABLE_EXTERNAL_SIGNER=ON -DCMAKE_INSTALL_PREFIX=$BASE_OUTDIR"
+BITCOIN_CONFIG_ALL="$BITCOIN_CONFIG_ALL -DCMAKE_INSTALL_PREFIX=$BASE_OUTDIR"
 
 if [[ "${RUN_TIDY}" == "true" ]]; then
   BITCOIN_CONFIG_ALL="$BITCOIN_CONFIG_ALL -DCMAKE_EXPORT_COMPILE_COMMANDS=ON"