Remove the syscall sandbox

After initially being merged in #20487, it's no-longer clear that an
internal syscall sandboxing mechanism is something that Bitcoin Core
should have/maintain, especially when compared to better
maintained/supported alterantives, i.e firejail.

Note that given where it's used, the sandbox also gets dragged into the
kernel.

There is some related discussion in #24771.

This should not require any sort of deprecation, as this was only ever
an opt-in, experimental feature.

Closes #24771.

ci/test/00_setup_env_i686_multiprocess.sh

@@ -15,4 +15,3 @@ export GOAL="install"
 export BITCOIN_CONFIG="--enable-debug CC='clang -m32' CXX='clang++ -m32' \
 LDFLAGS='--rtlib=compiler-rt -lgcc_s' CPPFLAGS='-DBOOST_MULTI_INDEX_ENABLE_SAFE_MODE'"
 export TEST_RUNNER_ENV="BITCOIND=bitcoin-node"
-export TEST_RUNNER_EXTRA="--nosandbox"

ci/test/00_setup_env_native_valgrind.sh

@@ -11,7 +11,7 @@ export CONTAINER_NAME=ci_native_valgrind
 export PACKAGES="valgrind clang llvm libclang-rt-dev python3-zmq libevent-dev bsdmainutils libboost-dev libdb5.3++-dev libminiupnpc-dev libnatpmp-dev libzmq3-dev libsqlite3-dev"
 export USE_VALGRIND=1
 export NO_DEPENDS=1
-export TEST_RUNNER_EXTRA="--nosandbox --exclude feature_init,rpc_bind,feature_bind_extra"  # Excluded for now, see https://github.com/bitcoin/bitcoin/issues/17765#issuecomment-602068547
+export TEST_RUNNER_EXTRA="--exclude feature_init,rpc_bind,feature_bind_extra"  # Excluded for now, see https://github.com/bitcoin/bitcoin/issues/17765#issuecomment-602068547
 export GOAL="install"
 # Temporarily pin dwarf 4, until using Valgrind 3.20 or later
 export BITCOIN_CONFIG="--enable-zmq --with-incompatible-bdb --with-gui=no CC=clang CXX=clang++ CFLAGS='-gdwarf-4' CXXFLAGS='-gdwarf-4'"  # TODO enable GUI