refactor: remove in-code warning suppression

Should no-longer be needed post #27872. If it is, then
suppress-external-warnings should be fixed.

src/test/system_tests.cpp

@@ -7,16 +7,7 @@
 #include <univalue.h>
 
 #ifdef ENABLE_EXTERNAL_SIGNER
-#if defined(__GNUC__)
-// Boost 1.78 requires the following workaround.
-// See: https://github.com/boostorg/process/issues/235
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wnarrowing"
-#endif
 #include <boost/process.hpp>
-#if defined(__GNUC__)
-#pragma GCC diagnostic pop
-#endif
 #endif // ENABLE_EXTERNAL_SIGNER
 
 #include <boost/test/unit_test.hpp>