validation: Remove incorrect safe mode addition to native_qt5

The merge commit incorrectly added BOOST_MULTI_INDEX_ENABLE_SAFE_MODE to
native_qt5.sh. This is not faithful to the Bitcoin backport because:

- Bitcoin's native_qt5.sh has --enable-debug, so it previously had safe mode enabled
- Dash's native_qt5.sh does NOT have --enable-debug, so it never had safe mode enabled
- The Bitcoin PR only re-enabled safe mode on builds that previously had it

The previous validation fixes (9ab5da6) correctly added safe mode to the 3 Dash
CI files that use --enable-debug or --enable-fuzz. This fix removes the incorrect
addition to native_qt5, making the backport faithful to Bitcoin's intent.

Author: Claude Code

ci/test/00_setup_env_native_qt5.sh

@@ -15,4 +15,4 @@ export RUN_UNIT_TESTS_SEQUENTIAL="true"
 export RUN_UNIT_TESTS="false"
 export GOAL="install"
 export PREVIOUS_RELEASES_TO_DOWNLOAD="v0.12.1.5 v0.15.0.0 v0.16.1.1 v0.17.0.3 v18.2.2 v19.3.0 v20.0.1"
-export BITCOIN_CONFIG="--enable-zmq --with-libs=no --enable-reduce-exports --disable-fuzz-binary LDFLAGS=-static-libstdc++ --with-boost-process CPPFLAGS='-DBOOST_MULTI_INDEX_ENABLE_SAFE_MODE'"
+export BITCOIN_CONFIG="--enable-zmq --with-libs=no --enable-reduce-exports --disable-fuzz-binary LDFLAGS=-static-libstdc++ --with-boost-process"