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

Author: fanquake

ci/test/00_setup_env_arm.sh

@@ -25,4 +25,4 @@ export RUN_FUNCTIONAL_TESTS=false
 export GOAL="install"
 # -Wno-psabi is to disable ABI warnings: "note: parameter passing for argument of type ... changed in GCC 7.1"
 # This could be removed once the ABI change warning does not show up by default
-export BITCOIN_CONFIG="--enable-glibc-back-compat --enable-reduce-exports CXXFLAGS=-Wno-psabi"
+export BITCOIN_CONFIG="--enable-reduce-exports CXXFLAGS=-Wno-psabi"

ci/test/00_setup_env_native_nowallet.sh

@@ -11,4 +11,4 @@ export DOCKER_NAME_TAG=ubuntu:18.04  # Use bionic to have one config run the tes
 export PACKAGES="python3-zmq clang-5.0 llvm-5.0"  # Use clang-5 to test C++17 compatibility, see doc/dependencies.md
 export DEP_OPTS="NO_WALLET=1"
 export GOAL="install"
-export BITCOIN_CONFIG="--enable-glibc-back-compat --enable-reduce-exports CC=clang-5.0 CXX=clang++-5.0"
+export BITCOIN_CONFIG="--enable-reduce-exports CC=clang-5.0 CXX=clang++-5.0"

ci/test/00_setup_env_native_qt5.sh

@@ -15,5 +15,5 @@ export RUN_UNIT_TESTS_SEQUENTIAL="true"
 export RUN_UNIT_TESTS="false"
 export GOAL="install"
 export PREVIOUS_RELEASES_TO_DOWNLOAD="v0.15.2 v0.16.3 v0.17.2 v0.18.1 v0.19.1 v0.20.1"
-export BITCOIN_CONFIG="--enable-zmq --with-libs=no --with-gui=qt5 --enable-glibc-back-compat --enable-reduce-exports
+export BITCOIN_CONFIG="--enable-zmq --with-libs=no --with-gui=qt5 --enable-reduce-exports
 --enable-debug --disable-fuzz-binary  CFLAGS=\"-g0 -O2 -funsigned-char\" CXXFLAGS=\"-g0 -O2 -funsigned-char\""

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