Cannot delete as-path prepend from policy when it contains more than one AS
Configuration
```
vyos@vyos# set policy route-map BGP-OUT rule 20 action 'permit'
[edit]
'yos@vyos# set policy route-map BGP-OUT rule 20 set as-path prepend '65999 65999
[edit]
vyos@vyos# commit
```
The error occurs after deleting an as-path prepend
```
vyos@vyos# delete policy route-map BGP-OUT rule 20 set
[edit]
vyos@vyos# commit
Failed to execute route-map BGP-OUT permit 20 no set as-path prepend
65999 65999 exit "route-map BGP-OUT permit 20 -- no set as-path prepend
65999 65999 -- exit" we failed to remove this command % Unknown command:
no set as-path prepend 65999 65999
[[policy]] failed
Commit failed
[edit]
```
If we use one AS in as-path prepend, everything works.
```
vyos@vyos# set policy route-map BGP-OUT rule 20 set as-path prepend '65999'
[edit]
vyos@vyos# set policy route-map BGP-OUT rule 20 action permit
[edit]
vyos@vyos# commit
[edit]
vyos@vyos# delete policy route-map BGP-OUT rule 20 set
[edit]
vyos@vyos# commit
[edit]
vyos@vyos#
```