configure: drop -Werror from the C++ warning probe

0ce413af9c gave check_disable_warning() a C++ checks at a time when
unknown_warning_flags was a single set probed with test_cflags, so it
said nothing about what the C++ compiler accepts. The -Werror was added
as a glue that reject C only flags from C++ compilation and probe time.
However, after 005510501e made those flags per language and
unknown_warning_cxxflags is now probed with test_cxxflags, so the Werror
is no longer relevant.

This is not only cosmetic change, because -Werror itself is not valid
flag for MSVC, all tests were failing there.

Signed-off-by: Kacper Michajłow <kasper93@gmail.com>

configure

@@ -7791,7 +7791,7 @@ check_warning(){
     warning_flag=$1
     flag_to_add=${2:-$1}
     test_cflags $unknown_warning_cflags $warning_flag && add_cflags $flag_to_add
-    test_cxxflags -Werror $unknown_warning_cxxflags $warning_flag && add_cxxflags $flag_to_add
+    test_cxxflags $unknown_warning_cxxflags $warning_flag && add_cxxflags $flag_to_add
 }
 
 check_disable_warning(){