scripts: fix GLUON_SITE_CONFIG

check_site.sh and site.sh were setting GLUON_SITE_CONFIG on their own
even though the environment variable was already exported.

scripts/check_site.sh

 #!/bin/sh
 
-export GLUON_SITE_CONFIG="$GLUONDIR/site/site.conf"
-
 SITE_CONFIG_LUA=packages/gluon/gluon/gluon-core/files/usr/lib/lua/gluon/site_config.lua
 CHECK_SITE_LIB=scripts/check_site_lib.lua
 

scripts/site.sh

 #!/bin/sh
 
-export GLUONDIR="$(dirname "$0")/.."
-export GLUON_SITE_CONFIG="$GLUONDIR/site/site.conf"
-
 SITE_CONFIG_LUA=packages/gluon/gluon/gluon-core/files/usr/lib/lua/gluon/site_config.lua
 
 "$GLUONDIR"/openwrt/staging_dir/host/bin/lua -e "print(assert(dofile(os.getenv('GLUONDIR') .. '/${SITE_CONFIG_LUA}').$1))" 2>/dev/null