Duplicate "Not found" notes from ExtractXliff

[papeh]

When running ExtractXliff.exe with --base-xliff, each string not found in the base Xliff is tagged with <note xml:lang="en">Not found in static scan of compiled code (version 0.0.0)</note>. This is expected.

If ExtractXliff.exe is run again on the same --base-xliff, another "not found" note is added for each run. This clutters the file.

Two requests:

1. Don't exceed one "not found" note per string
2. Remove "not found" notes if a string is found in a subsequent extraction

These can both be accomplished by stripping "not found" notes from the Xliff before extraction.