[mgmt-framework]: Removing explicit libyang plugin path settings (#4144)

Removing explicit libyang plugin path settings. Libyang debian package installs the extensions in standard location.

Libyang debian package installs the extensions in standard location as per build time configuration (install prefix is set in patch/libyang.patch). If plugin directory is not set, libyang explicitly searches the plugins in LYEXT_PLUGINS_DIR which is populated during build. The plugins are installed in LYEXT_PLUGINS_DIR itself through debian package installation.

$ grep -nr LYEXT_PLUGINS_DIR src/
src/plugin_config.h.in:18:#define LYEXT_PLUGINS_DIR "@EXTENSIONS_PLUGINS_DIR_MACRO@" /**< directory with YANG extension plugins */
src/plugins.c:420: pluginsdir = LYEXT_PLUGINS_DIR;

dockers/docker-sonic-mgmt-framework/rest-server.sh

@@ -46,7 +46,5 @@ echo "REST_SERVER_ARGS = $REST_SERVER_ARGS"
 
 
 export CVL_SCHEMA_PATH=/usr/sbin/schema
-export LIBYANG_EXTENSIONS_PLUGINS_DIR=/usr/lib/x86_64-linux-gnu/libyang/extensions
-export LIBYANG_USER_TYPES_PLUGINS_DIR=/usr/lib/x86_64-linux-gnu/libyang/user_types
 
 exec /usr/sbin/rest_server ${REST_SERVER_ARGS}