Merge #21610: build: remove -Wdeprecated-register from NOWARN flags

1a011b3 build: remove -Wdeprecated-register from NOWARN flags (fanquake)

Pull request description:

  The `register` keyword was deprecated in C++11, and [removed in C++17](https://en.cppreference.com/w/cpp/keyword/register). Now that we require C++17, we shouldn't have to suppress warnings for a non-existent feature.

ACKs for top commit:
  sipa:
    utACK 1a011b3
  hebasto:
    ACK 1a011b3

Tree-SHA512: 7546b3870fe819507deea57d5c3179bc5debd4513df41b3f74d191995116b1507ff2a77cf2081ac32871194d4e5c2a2913c668c56244dff39853a5da18194f9f

Author: MarcoFalke

configure.ac

@@ -472,7 +472,6 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wunused-parameter],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-unused-parameter"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wself-assign],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-self-assign"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wunused-local-typedef],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-unused-local-typedef"],,[[$CXXFLAG_WERROR]])
-  AX_CHECK_COMPILE_FLAG([-Wdeprecated-register],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-deprecated-register"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wimplicit-fallthrough],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-implicit-fallthrough"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wdeprecated-copy],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-deprecated-copy"],,[[$CXXFLAG_WERROR]])
 fi