COMP: Remove default -Wno-deprecated from the ITK warning flags

-Wno-deprecated was added in 2009 (a33ca7a, Bill Lorensen) inside a
CMAKE_COMPILER_IS_GNUCXX guard to silence GCC warnings about deprecated
C++ standard headers (<strstream>, <ext/hash_map>, <backward/*>). Those
headers are long gone. In 2011 (8a135bc) the flag was folded into the
cross-compiler cxx_flags warning set, so it has since applied to all
compilers, not just GCC.

The [[deprecated]] attribute did not exist until C++14, so this flag was
never intended to mask attribute-based deprecations. On Clang,
-Wno-deprecated also disables -Wdeprecated-declarations, so it silently
hides [[deprecated]] API warnings on Clang while GCC (where the two are
independent) still emits them -- an inconsistent, surprising default that
hides ITK's own and vendored deprecations from Clang builds.

Remove it so deprecation warnings surface uniformly across compilers.
ITK_CXX_WARNING_FLAGS remains available to override the warning set.

Author: hjmjohnson

CMake/ITKSetStandardCompilerFlags.cmake

@@ -133,7 +133,6 @@ function(check_compiler_warning_flags c_warning_flags_var cxx_warning_flags_var)
   # Check this list on C++ compiler only
   set(
     cxx_flags
-    -Wno-deprecated
     -Wno-invalid-offsetof
     -Wno-undefined-var-template # suppress invalid warning when explicitly instantiated in another translation unit
     -Woverloaded-virtual