Delete obsolete Solaris compat header file stdlib.h

This drops function getexecname() redirection.

Historically getexecname() is a compatibility definition. Since
openzfs has its own implementation of function getexecname() in libspl
and has been merged into base, the compat header file stdlib.h is
no longer needed and should not be used.

Also without this fix libspl will end up an incompatible version of
getprogname() with libc. In particular, if zfs is enabled, programs
such as pgrep in /rescue can be wrongly statically linked with libspl
and will not function properly.

PR: 269738
Reviewed by: markj
Fixes: 9e5787d2284e Merge OpenZFS support in to HEAD
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D38733