build: Drop option for disabling hardening

Building unhardened executables is not a supported use case that should
be maintained and those that want unhardened executables can still
override them by appending disable flags.

For example:

cmake -B build -DAPPEND_CPPFLAGS='-U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=0 -fno-stack-protector -fcf-protection=none -fno-stack-clash-protection' -DAPPEND_LDFLAGS='-Wl,-z,lazy -Wl,-z,norelro -Wl,-z,noseparate-code'

CMakePresets.json

@@ -77,7 +77,6 @@
         "BUILD_UTIL_CHAINSTATE": "ON",
         "BUILD_WALLET_TOOL": "ON",
         "ENABLE_EXTERNAL_SIGNER": "ON",
-        "ENABLE_HARDENING": "ON",
         "ENABLE_WALLET": "ON",
         "WARN_INCOMPATIBLE_BDB": "OFF",
         "WITH_BDB": "ON",