remove index length raise
remove index length raise

because a) it's not getting hit (it's being prepended to SchemaStatements,
which has already been included into PostgreSQLAdapter, so PostgreSQLAdapter
doesn't get it), and b) Rails already does the same thing in add_index_options

Change-Id: Id6681c200f98812a8781fc99d6108e2a8e7e7536
Reviewed-on: https://gerrit.instructure.com/106778
Tested-by: Jenkins
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

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