strbuf: remove unreferenced strbuf_write_fd method.

strbuf: remove unreferenced strbuf_write_fd method.

strbuf_write_fd was only used in bugreport.c. Since that file now uses
write_in_full, this method is no longer needed. In addition, strbuf_write_fd
did not guard against exceeding MAX_IO_SIZE for the platform, nor
provided error handling in the event of a failure if only partial data
was written to the file descriptor. Since already write_in_full has this
capability and is in general use, it should be used instead. The change
impacts strbuf.c and strbuf.h.

Signed-off-by: Randall S. Becker <redacted>
Signed-off-by: Junio C Hamano <redacted>