Remove unused EventLoopTimer.Arm return type (#23765)

## Summary

The `EventLoopTimer.Arm` result from `EventLoopTimer.fire()` was being
ignored at both call sites. This PR removes the unused return type and
simplifies the code.

## Changes

- Changed `EventLoopTimer.fire()` to return `void` instead of `Arm`
- Updated all 15 timer callback functions to return `void`
- Removed the `Arm` type definition
- Simplified the `drainTimers()` loop that was ignoring the return value
- Updated both call sites in `Timer.zig`

## Details

The `.rearm` functionality was unused - timers that need to reschedule
themselves (like DNS resolver) handle this by calling
`addTimer()`/`update()` directly rather than relying on the return
value.

This change removes:
- The `Arm` union enum type (3 lines)
- All `return .disarm` and `return .{ .rearm = ... }` statements
- The switch statement in `drainTimers()` that did nothing with the
return value

Net result: **-58 lines** of dead code removed.

## Testing

- [x] Bun builds successfully with `bun bd`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>

src/sql/mysql/js/JSMySQLConnection.zig

@@ -106,18 +106,18 @@ pub fn resetConnectionTimeout(this: *@This()) void {
     this.#vm.timer.insert(&this.timer);
 }
 
-pub fn onConnectionTimeout(this: *@This()) bun.api.Timer.EventLoopTimer.Arm {
+pub fn onConnectionTimeout(this: *@This()) void {
     this.timer.state = .FIRED;
 
     if (this.#connection.isProcessingData()) {
-        return .disarm;
+        return;
     }
 
-    if (this.#connection.status == .failed) return .disarm;
+    if (this.#connection.status == .failed) return;
 
     if (this.getTimeoutInterval() == 0) {
         this.resetConnectionTimeout();
-        return .disarm;
+        return;
     }
 
     switch (this.#connection.status) {
@@ -135,14 +135,12 @@ pub fn onConnectionTimeout(this: *@This()) bun.api.Timer.EventLoopTimer.Arm {
         },
         .disconnected, .failed => {},
     }
-    return .disarm;
 }
 
-pub fn onMaxLifetimeTimeout(this: *@This()) bun.api.Timer.EventLoopTimer.Arm {
+pub fn onMaxLifetimeTimeout(this: *@This()) void {
     this.max_lifetime_timer.state = .FIRED;
-    if (this.#connection.status == .failed) return .disarm;
+    if (this.#connection.status == .failed) return;
     this.failFmt(error.LifetimeTimeout, "Max lifetime timeout reached after {}", .{bun.fmt.fmtDurationOneDecimal(@as(u64, this.max_lifetime_interval_ms) *| std.time.ns_per_ms)});
-    return .disarm;
 }
 fn setupMaxLifetimeTimerIfNecessary(this: *@This()) void {
     if (this.max_lifetime_interval_ms == 0) return;