execute: drop dead code

execute: drop dead code

On several platforms, I noticed this compile warning:
gcc -DHAVE_CONFIG_H -I. -I../../gllib -I..  -DGNULIB_STRICT_CHECKING=1   -Wall -MT execute.o -MD -MP -MF .deps/execute.Tpo -c -o execute.o ../../gllib/execute.c
../../gllib/execute.c:64:1: warning: ‘nonintr_close’ defined but not used [-Wunused-function]
../../gllib/execute.c:77:1: warning: ‘nonintr_open’ defined but not used [-Wunused-function]

The warning first surfaced when commit bdaf232 (Nov 2012) finally
pointed out that these wrappers were no longer needed on posix-y
systems, although the code has been unused since commit d629f6d
(Jan 2009) which removed all use of open()/close() in favor of
posix_spawn() instead.  The only platform remaining where the
wrappers are used (and no warnings issued) is mingw, but according
to Microsoft's documentation [1] at the time of this patch, mingw's
libc never fails open or close with EINTR; not to mention the fact
that the documentation of the wrapper mentions it is primarily
useful for SIGSTOP, a concept not present on mingw.

[1]http://msdn.microsoft.com/en-us/library/z0kc8e3z%28v=vs.80%29.aspx

* lib/execute.c (nonintr_close, nonintr_open): Delete.

Signed-off-by: Eric Blake <eblake@redhat.com>