chore(SpecificGroups/Alternating/Simple): remove a convert

[b-mehta]

As mentioned here https://leanprover.zulipchat.com/#narrow/channel/287929-mathlib4/topic/support.20of.20permutations.20on.20Type.20instead.20of.20Fintype/with/593547821

I misdiagnosed the issue in that thread, and this PR only fixes one of the issues.
The "issue" in this case was that the lemma cycleType_ofSubtype contains a non-variable instance of Fintype (Subtype p), which means that when applying the lemma, the "wrong" instance could be picked. The correct pattern is that if a lemma requires Fintype _, then it should be stated explicitly in the type, even if that instance is derivable from other hypotheses. (An existing example is https://leanprover-community.github.io/mathlib4_docs/Mathlib/Data/Fintype/Sets.html#Set.toFinset_union).

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[github-actions]

PR summary 260979c9dd

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

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No changes to strong technical debt.

No changes to weak technical debt.

[tb65536]

Nice, thanks!