Drop vsprintf.c

Drop vsprintf.c

Drop custom vfprintf implementation that was disabled by default.
Apparently nobody tests strace with this vfprintf enabled, otherwise
multiple uses of format specifiers not supported by this custom vfprintf
would not left unnoticed.

The GNU C library is not the only libc available, so those who want
faster implementations of libc functions are encouraged to try building
strace with other libc implementations.

* vsprintf.c: Remove.
* Makefile.am (strace_SOURCES): Remove vsprintf.c.
* defs.h (USE_CUSTOM_PRINTF, strace_vfprintf): Remove.
* strace.c (tprintf): Replace strace_vfprintf with vfprintf.