Remove the NO_SECURITY compile-time option.

It's had its day. It was there to support pre-WinNT platforms, on
which the security APIs don't exist - but more specifically, it was
there to support _build tools_ that only knew about pre-WinNT versions
of Windows, so that you couldn't even compile a program that would
_try_ to refer to the interprocess security APIs.

But we don't support those build systems any more in any case: more
recent changes like the assumption of (most of) C99 will have stopped
this code from building with compilers that old. So there's no reason
to clutter the code with backwards compatibility features that won't
help.

I left NO_SECURITY in place during the CMake migration, so that _just_
in case it needs resurrecting, some version of it will be available in
the git history. But I don't expect it to be needed, and I'm deleting
the whole thing now.

The _runtime_ check for interprocess security libraries is still in
place. So PuTTY tools built with a modern toolchain can still at least
try to run on the Win95/98/ME series, and they should detect that
those system DLLs don't exist and proceed sensibly in their absence.
That may also be a thing to throw out sooner or later, but I haven't
thrown it out as part of this commit.

cmake/platforms/windows.cmake

@@ -1,16 +1,5 @@
 set(PLATFORM_SUBDIRS windows)
 
-# I copied this over from the pre-CMake build system just to prove it
-# still worked, but I should probably remove it now, together with all
-# the #ifdefs that depend on it.
-#
-# Rationale: it was there so that you could do dev builds of PuTTY on
-# compilers designed for the pre-NT single-user versions of Windows
-# (Win95, Win98 etc). But we're not supporting those development
-# environments any more!
-set(PUTTY_NO_SECURITY OFF
-  CACHE BOOL "OBSOLETE AND DANGEROUS - DO NOT DEFINE! \
-Build PuTTY without any use of the Windows security APIs.")
 set(PUTTY_MINEFIELD OFF
   CACHE BOOL "Build PuTTY with its built-in memory debugger 'Minefield'")
 set(PUTTY_GSSAPI ON