mountpoints: remove useless default mountpoints

• Remove the objects not shared by saam
• Change the default mode to 'ro' for the space mountpoint

Related to T3899

Test Plan

• workers:

*******************************************
- Exec[create /srv/softwareheritage/objects]
*******************************************
- File[/srv/softwareheritage/objects]
*******************************************
- Mount[/srv/softwareheritage/objects]
*******************************************
  Mount[/srv/storage/space] =>
   parameters =>
     options =>
      - rw,soft,intr,rsize=8192,wsize=8192,noauto,x-systemd.automount,x-systemd.device-timeout=10
      + ro,soft,intr,rsize=8192,wsize=8192,noauto,x-systemd.automount,x-systemd.device-timeout=10
*******************************************
*** End octocatalog-diff on worker01.softwareheritage.org

• saam:

*** Running octocatalog-diff on host saam.internal.softwareheritage.org
I, [2022-02-03T11:31:46.556785 #302666]  INFO -- : Catalogs compiled for saam.internal.softwareheritage.org
I, [2022-02-03T11:31:47.045870 #302666]  INFO -- : Diffs computed for saam.internal.softwareheritage.org
I, [2022-02-03T11:31:47.045912 #302666]  INFO -- : No differences
*** End octocatalog-diff on saam.internal.softwareheritage.org

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Migrated from D7068 (view on Phabricator)