BSON writer: drop the SFINAE guard that Visual Studio 2017 cannot deduce

write_bson_document_single_pass() was a member template enabled by
is_detected<sink_written_t, SinkType>. MSVC 19.16 (Visual Studio 2017)
fails to deduce it through the class-scope alias template (C2672, C2783)
when it compiles to_bson() and to_bon8(), so every Visual Studio 2017
job on AppVeyor fails since #5979.

The guard is not needed: as a non-template member of the class template,
the function is only instantiated when called, and its only caller is
the write_bson_document() overload for sinks that have written(), which
already calls oa.written() and oa.rewind() itself.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N5NasAgbDGG28MqXjQvLqp
Signed-off-by: Claude <noreply@anthropic.com>

include/nlohmann/detail/output/binary_writer.hpp

@@ -2422,7 +2422,6 @@ class binary_writer
             nothing large has been written then, and the caller writes the
             document in two passes, which reports that error
     */
-    template<typename SinkType = OutputSinkType, typename std::enable_if<is_detected<sink_written_t, SinkType>::value, int>::type = 0>
     bool write_bson_document_single_pass(const BasicJsonType& document)
     {
         const std::size_t start = oa.written();

single_include/nlohmann/json.hpp

@@ -30394,7 +30394,6 @@ class binary_writer
             nothing large has been written then, and the caller writes the
             document in two passes, which reports that error
     */
-    template<typename SinkType = OutputSinkType, typename std::enable_if<is_detected<sink_written_t, SinkType>::value, int>::type = 0>
     bool write_bson_document_single_pass(const BasicJsonType& document)
     {
         const std::size_t start = oa.written();