enable PostAsync_ReuseRequestContent_Success on Unix (#2118)

src/libraries/Common/tests/System/Net/Http/HttpClientHandlerTest.cs

@@ -2216,7 +2216,6 @@ public async Task PostAsync_Redirect_LargePayload_Helper(Configuration.Http.Remo
 
         [OuterLoop("Uses external server")]
         [Theory, MemberData(nameof(RemoteServersMemberData))]
-        [ActiveIssue("https://github.com/dotnet/corefx/issues/31104", TestPlatforms.AnyUnix)]
         public async Task PostAsync_ReuseRequestContent_Success(Configuration.Http.RemoteServer remoteServer)
         {
             const string ContentString = "This is the content string.";