Revert "Add baseimage for windows 1809"

[abayer]

Changes

This reverts commit 99ab40c - PR #5361

nanoserver:1809 and nanoserver:ltsc2019 are actually the same iamge, causing combine to fail.

/kind cleanup

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[abayer]

/assign @imjasonh @vdemeester @lbernick @jerop @pritidesai

[imjasonh]

/lgtm

[pritidesai]

sounds good, thank you @abayer for cleaning up 👍

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