mgr/cephadm/osd: update removal report immediately

mgr/cephadm/osd: update removal report immediately

Currently, the removal report is updated after entering the main serve()
loop. This tiny window makes clients (like Dashboard) fail to poll the
result. Refresh the report data immediately after scheduling OSD for
removal.

The report structure is changed from Dict to Set because:
- The key is `OSDRemoval` instance, which make it hard to use.
- More consistent with orchestrator interfaces: most calls return a list.

Signed-off-by: Kiefer Chang <kiefer.chang@suse.com>