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.

src/Makefile.am

@@ -313,7 +313,6 @@ BITCOIN_CORE_H = \
   util/sock.h \
   util/spanparsing.h \
   util/string.h \
-  util/syscall_sandbox.h \
   util/syserror.h \
   util/thread.h \
   util/threadinterrupt.h \
@@ -741,7 +740,6 @@ libbitcoin_util_a_SOURCES = \
   util/spanparsing.cpp \
   util/strencodings.cpp \
   util/string.cpp \
-  util/syscall_sandbox.cpp \
   util/time.cpp \
   util/tokenpipe.cpp \
   $(BITCOIN_CORE_H)
@@ -976,7 +974,6 @@ libbitcoinkernel_la_SOURCES = \
   util/serfloat.cpp \
   util/strencodings.cpp \
   util/string.cpp \
-  util/syscall_sandbox.cpp \
   util/syserror.cpp \
   util/thread.cpp \
   util/threadnames.cpp \