Clean up delete widget functions in CommunalWidgetRepository

This follow-up change (after ag/26073820) is to consolidate the two
deleteFromHost() and deleteFromDb() into one deleteWidget() function
for simplicity.

Still use DB as the source of truth to check whether the id requested
to be deleted exists then delete. If returning false, nothing will be
deleted from DB or widget host.

Bug: b/323265998
Test: atest CommunalWidgetRepositoryImplTest
Flag: ACONFIG com.android.systemui.communal_hub STAGING
Change-Id: Ied2c69bcdfad7007bbcb6762cb96ff2239752729