Fix unused variable warnings in tests

Signed-off-by: Damian Jarek <damian.jarek93@gmail.com>

test/beast/core/basic_stream.cpp

@@ -1204,7 +1204,6 @@ public:
         }
 
         {
-            error_code ec;
             stream_type s(ioc);
             async_teardown(role_type::server, s,
                 [](error_code)

test/beast/core/file_test.hpp

@@ -72,7 +72,6 @@ test_file()
     auto const remove =
         [](fs::path const& path)
         {
-            error_code ec;
             fs::remove(path);
         };
 

test/beast/core/flat_stream.cpp

@@ -148,7 +148,6 @@ public:
             BEAST_EXPECT(buffer_bytes(bs) <=
                 detail::flat_stream_base::max_size);
             flat_stream<test::stream> s(ioc);
-            error_code ec;
             s.async_write_some(bs,
                 [](error_code, std::size_t)
                 {