Remove unused missing/snprintf.c. [skip ci]

Remove unused missing/snprintf.c. [skip ci]

The file could be compiled on demand using "make snprintf.o", although
with many warnings, but tcpdump build would never use the file
regardless of the variety of the build.

Some of the code there has been commented out since commit 45aef49 in
2001, and the rest became unused after commit 1ed63b5 in 2019.