default_dashboard was deprecated a long ago in Horizon[1] and
Having that parameter in local_settings triggers the following warning
message.
```
WARNING:root:"dashboards" and "default_dashboard" in (local_)settings
is DEPRECATED now and may be unsupported in some future release.
The preferred way to specify the order of dashboards and the default
dashboard is the pluggable dashboard mechanism (in ...).
```
This change removes that parameter hard-coded in the template file to
get rid of the warning message. Currently the project dashboard is
shown first in Horizon by default so removing the parameter does not
cause any change in behavior.
This also removes these deprecated parameters from the template file
used in unit tests.
[1] 75bc2e6aa86d31f60288021694699302b3873af3
Closes-Bug: #1983390
Change-Id: I8cd5346a553db9ba9f6bdf8ffdd9e7d89d16c4be
57905ff386