CI: fail to fetch dashboard

[membphis]

+ git clone -b master https://github.com/apache/apisix.git /tmp/apisix
Cloning into '/tmp/apisix'...
+ cd /tmp/apisix
+ git submodule init
+ git submodule update
+ cd dashboard
/bin/sh: cd: line 1: can't cd to dashboard: No such file or directory

we need to change the script, fetch it from https://github.com/apache/apisix-dashboard

[moonming]

We can remove dashboard until new dashboard released