Remove links to nonexistent migration docs

docs/reference/migration/index.asciidoc

@@ -17,17 +17,3 @@ As a general rule:
 See <<setup-upgrade>> for more info.
 --
 include::migrate_5_0.asciidoc[]
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-include::migrate_2_3.asciidoc[]
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-include::migrate_2_2.asciidoc[]
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-include::migrate_2_1.asciidoc[]
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-include::migrate_2_0.asciidoc[]
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-include::migrate_1_6.asciidoc[]
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-include::migrate_1_4.asciidoc[]
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-include::migrate_1_0.asciidoc[]