[fix] Fix backup & restore system

See https://github.com/YunoHost-Apps/dokuwiki_ynh/pull/37#pullrequestreview-134457098
and https://github.com/YunoHost/yunohost/pull/489#issuecomment-396854686

scripts/backup

@@ -6,12 +6,7 @@
 # IMPORT GENERIC HELPERS
 #=================================================
 
-if [ ! -e _common.sh ]; then
-	# Get the _common.sh file if it's not in the current directory
-	cp ../settings/scripts/_common.sh ./_common.sh
-	chmod a+rx _common.sh
-fi
-source _common.sh
+source ../settings/scripts/_common.sh
 source /usr/share/yunohost/helpers
 
 #=================================================

scripts/restore

@@ -6,12 +6,7 @@
 # IMPORT GENERIC HELPERS
 #=================================================
 
-if [ ! -e _common.sh ]; then
-	# Get the _common.sh file if it's not in the current directory
-	cp ../settings/scripts/_common.sh ./_common.sh
-	chmod a+rx _common.sh
-fi
-source _common.sh
+source ../settings/scripts/_common.sh
 source /usr/share/yunohost/helpers
 
 #=================================================