Remove assigned-but-never-read fields and unused catch variable

[anagnorisis2peripeteia]

Part of #38 (dead-code audit).

What

Remove three analyzer-flagged dead values that are never read:

• Mouse.currentToggleGyroMouse / currentToggleGyroStick (CS0414) — written in
  ResetToggleGyroModes() but never read anywhere. Removed the fields and their two writes.
  (currentToggleGyroControls, which is read, is kept.)
• The unused exception variable in ScpUtil's Profiles.xml load catch (CS0168):
  catch (InvalidOperationException e) -> catch (InvalidOperationException).

No behavior change.

How verified

Unlike removing an unreferenced declaration or an unused using (where "it compiles" already
implies "safe"), removing an assigned-but-never-read value could silently drop a side effect —
so each was checked by hand first: both gyro fields are only ever assigned = false (no side
effect), and dropping an unused catch identifier doesn't change which exception is caught. Then
compiled:

x64 Release publish : PUBLISH_X64_EXIT=0
x86 Release publish : PUBLISH_X86_EXIT=0
DS4WindowsTests     : Passed 11, Failed 1, Total 12

The single test failure (CheckSettingsRead) is a pre-existing en-GB date-locale snapshot
mismatch, identical on untouched upstream/main.

Scope

Deliberately limited to values whose removal is provably side-effect-free. Two further
assigned-never-read candidates — ControlService.useRumble (duplicate near-identical code
blocks) and DS4Device.outputPendCount (private base field vs subclass shadowing) — are held
back for a traced follow-up rather than a mechanical removal.

Authored with Claude (Opus 4.8).

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Review history: https://gist.github.com/00b6e96ba78cc14bc88376b161925ded