Remove redundant availability guard (#2379)

Motivation:

EventLoopTest has an availability annotation for the various Darwin
platforms. This availability check is repeated in a few tests which
yields warnings.

Modifications:

Remove the class level availability guard.

Result:

Warning free.

Co-authored-by: Cory Benfield <lukasa@apple.com>

Tests/NIOPosixTests/EventLoopTest.swift

@@ -19,7 +19,6 @@ import Atomics
 import Dispatch
 import NIOConcurrencyHelpers
 
-@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 public final class EventLoopTest : XCTestCase {
 
     public func testSchedule() throws {
@@ -813,7 +812,7 @@ public final class EventLoopTest : XCTestCase {
 
         class Thing: @unchecked Sendable {
             private let deallocated: ConditionLock<Int>
-            
+
             init(_ deallocated: ConditionLock<Int>) {
                 self.deallocated = deallocated
             }