Improve Remove-IshBaselineItem by not providing version (currently used hardcoded 999999)

[OlegTokar]

Due to the hardcoded version in cmd-let implementetation
_baselineItemsToProcess[ishObject.IshRef].Add(new IshBaselineItem(ishObject.IshRef, LogicalId, "999999"));
it would fail in Tridion Docs 15.2 and up with:
[-1] An unexpected system failure occurred. The provided version number 999999 does not match the version '1' in the database. [1;SystemFailure]

ToDo:

• Improve cmd-let by not providing version number
• Don't pass version parameter at all to the Update call
• Add Pester tests for the cmd-let