build: Disable Wshadow warning

This warning was enabled by default in #8808 but it's a
[continuing](bitcoin/bitcoin#9911 (comment))
[source](bitcoin/bitcoin#10089 (comment)) of
[annoyance](bitcoin/bitcoin#9911 (comment)) for me
and other developers. I'm sick of sounding like a broken record, so disable it again.

Author: laanwj

configure.ac

@@ -227,7 +227,6 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wformat],[CXXFLAGS="$CXXFLAGS -Wformat"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wvla],[CXXFLAGS="$CXXFLAGS -Wvla"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wformat-security],[CXXFLAGS="$CXXFLAGS -Wformat-security"],,[[$CXXFLAG_WERROR]])
-  AX_CHECK_COMPILE_FLAG([-Wshadow],[CXXFLAGS="$CXXFLAGS -Wshadow"],,[[$CXXFLAG_WERROR]])
 
   ## Some compilers (gcc) ignore unknown -Wno-* options, but warn about all
   ## unknown options if any other warning is produced. Test the -Wfoo case, and