Drop the C shim for float formatting now that jsonpull is C++

json_print_one() reached fpfmt through dtoa_shortest(), a C-linkage
wrapper in text.cpp that strdup()ed the formatted string so the C
jsonpull could take it and free() it. jsonpull has been C++ since 4f26211,
so it can append the std::string directly and skip the copy, the
allocation, and the free.

That was the shim's only caller, so fpfmt/fpfmt.h goes away with it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014wJRAuhMninQE4wK2TUfuZ

fpfmt/fpfmt.h

@@ -1,14 +0,0 @@
-// C-callable shortest-float formatting, for jsonpull.
-// The implementation lives in text.cpp and calls fpfmt::dtoa().
-
-#ifdef __cplusplus
-extern "C" {
-#endif
-
-// dtoa_shortest returns a newly strdup()ed shortest decimal form of val.
-// The caller owns the result.
-char *dtoa_shortest(double val);
-
-#ifdef __cplusplus
-}
-#endif