guix: Drop code for the unsupported i686-linux-gnu host

Now GUIX build for the `i686-linux-gnu` host is broken, and there are no
plans to re-add it.

Author: hebasto

contrib/guix/libexec/build.sh

@@ -167,7 +167,6 @@ case "$HOST" in
     *linux*)
         glibc_dynamic_linker=$(
             case "$HOST" in
-                i686-linux-gnu)        echo /lib/ld-linux.so.2 ;;
                 x86_64-linux-gnu)      echo /lib64/ld-linux-x86-64.so.2 ;;
                 arm-linux-gnueabihf)   echo /lib/ld-linux-armhf.so.3 ;;
                 aarch64-linux-gnu)     echo /lib/ld-linux-aarch64.so.1 ;;
@@ -204,19 +203,12 @@ make -C depends --jobs="$JOBS" HOST="$HOST" \
                                    ${SOURCES_PATH+SOURCES_PATH="$SOURCES_PATH"} \
                                    ${BASE_CACHE+BASE_CACHE="$BASE_CACHE"} \
                                    ${SDK_PATH+SDK_PATH="$SDK_PATH"} \
-                                   i686_linux_CC=i686-linux-gnu-gcc \
-                                   i686_linux_CXX=i686-linux-gnu-g++ \
-                                   i686_linux_AR=i686-linux-gnu-ar \
-                                   i686_linux_RANLIB=i686-linux-gnu-ranlib \
-                                   i686_linux_NM=i686-linux-gnu-nm \
-                                   i686_linux_STRIP=i686-linux-gnu-strip \
                                    x86_64_linux_CC=x86_64-linux-gnu-gcc \
                                    x86_64_linux_CXX=x86_64-linux-gnu-g++ \
                                    x86_64_linux_AR=x86_64-linux-gnu-ar \
                                    x86_64_linux_RANLIB=x86_64-linux-gnu-ranlib \
                                    x86_64_linux_NM=x86_64-linux-gnu-nm \
                                    x86_64_linux_STRIP=x86_64-linux-gnu-strip \
-                                   qt_config_opts_i686_linux='-platform linux-g++ -xplatform bitcoin-linux-g++' \
                                    qt_config_opts_x86_64_linux='-platform linux-g++ -xplatform bitcoin-linux-g++' \
                                    FORCE_USE_SYSTEM_CLANG=1