remove --enable-glibc-back-compat from CI and docs

doc/build-unix.md

@@ -348,7 +348,7 @@ To build executables for ARM:
     make HOST=arm-linux-gnueabihf NO_QT=1
     cd ..
     ./autogen.sh
-    CONFIG_SITE=$PWD/depends/arm-linux-gnueabihf/share/config.site ./configure --enable-glibc-back-compat --enable-reduce-exports LDFLAGS=-static-libstdc++
+    CONFIG_SITE=$PWD/depends/arm-linux-gnueabihf/share/config.site ./configure --enable-reduce-exports LDFLAGS=-static-libstdc++
     make