[IMP] account_asset_management: performance improvement when recomputing the depreciation board

This commit removes an unncessary JOIN, and reduces the number
of computation of the depreciation values. It is not necessary to
recompute the depreciation values when computing the depreciation
board, only when posting depreciation lines.
Merge pull request #3 from acsone/7.0-asset-management-perf-ldm-sbi

[IMP] account_asset_management: performance improvement when recomputing...
@sbidoul  : thanks for this improvement.