[material_ui] Remove redundant disabledBorder check in InputDecorationThemeData

[MohanadAbdallah-mv]

Removes a duplicate condition in the == operator of InputDecorationThemeData.
The check other.disabledBorder == disabledBorder appeared twice ,This was noticed during the review of flutter/flutter#183582.
No behavior change since the duplicate was always true when reached.

List which issues are fixed by this PR. You must list at least one issue.
Port of flutter/flutter#187556

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Code Review

This pull request removes a redundant disabledBorder check from the equality operator (==) in InputDecorationThemeData within input_decorator.dart. It also adds a corresponding changelog entry. There are no review comments, and I have no feedback to provide.

[MohanadAbdallah-mv]

@Renzo-Olivares @QuncCccccc
This current PR is a port of flutter/flutter#187556