dirstate: inline the `from_p2_removed` logic
It is used internally for compatibilty with size used in the `v1` format, but
this is the only use. So we can simply inline it.
Differential Revision: https://phab.mercurial-scm.org/D11514
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed