depends: remove NO_HARDEN option

This only existed to workaround a (iirc libtool related) windows issue
that only occured when compiling without hardening. We no-longer use
libtool, and I can no-longer create the failure.

depends/README.md

@@ -127,7 +127,6 @@ The following can be set when running make: `make FOO=bar`
   resides in the `depends` directory, and the log file is printed out automatically in case
   of build error. After successful build log files are moved along with package archives
 - `LTO`: Enable options needed for LTO. Does not add `-flto` related options to *FLAGS.
-- `NO_HARDEN=1`: Don't use hardening options when building packages
 
 If some packages are not built, for example `make NO_WALLET=1`, the appropriate CMake cache
 variables will be set when generating the Bitcoin Core buildsystem. In this case, `-DENABLE_WALLET=OFF`.