remove last-modified timestamp from ASCIITLD.jflex [LUCENE-9926]

[asfimport]

Followup to #10962

Unfortunately this file seems to be generated daily and the date bumped even if no "real content" changed. So it is still causing many useless changes anytime regenerate runs.

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Migrated from LUCENE-9926 by Robert Muir (@rmuir), resolved Apr 19 2021
Linked issues:

• Add non-file properties to wrapped regenerate checksums [LUCENE-9933] #10972

[asfimport]

Adrien Grand (@jpountz) (migrated from JIRA)

Closing after the 9.0.0 release