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

On Clang, -Wno-deprecated subsumes -Wdeprecated-declarations and
hides [[deprecated]]/ITK_DEPRECATED API warnings; on GCC the two are
independent, so deprecation visibility differed by compiler. Removing
the flag surfaces these warnings uniformly. The narrower
-Wno-deprecated-copy keeps the -Wextra implicit-copy lint quiet in
headers (e.g. vendored GDCM) pending separate cleanup.
ITK_CXX_WARNING_FLAGS remains available to override the warning set.

Author: hjmjohnson

CMake/ITKSetStandardCompilerFlags.cmake

@@ -133,7 +133,7 @@ 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-deprecated-copy # implicit-copy lint suppressed; -Wdeprecated-declarations stays active
     -Wno-invalid-offsetof
     -Wno-undefined-var-template # suppress invalid warning when explicitly instantiated in another translation unit
     -Woverloaded-virtual