If for whatever reason we fail to import MR data,
subsequent run will fail as we try to insert duplicate data.
Instead of trying to recover, lets delete all and retry again.
The SQL queries and memory allocation in MergeRequests::RefreshService
is dominated by queries for Project and Route loads. On staging, the
absence of an inverse relationship caused Rails to make over 1100
extraneous SQL queries for the www-gitlab-com repository.
Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/49703
