libast/tm/tvsleep.c: remove assert.h use (re: 6e06e9c, 4e47f89)

Including <assert.h> after <ast.h> caused the build to fail on my
museum-grade PowerMac G5 with gcc 3.3. This may be fixed by
including <assert.h> (which is a non-AST system header) before
<ast.h> instead, which should not cause any problems.

However, the assert(3) macro is only used once to verify that the
tv pointer passed to tvsleep(3) is non-NULL. This is non-essential
as the pointer is dereferenced immediately after, and dereferencing
a null pointer reliably triggers a traceable segfault. None of the
other AST code uses assert(3), so let's just get rid instead.

This fixes the build on (at least) the system mentioned above.

src/lib/libast/tm/tvsleep.c

@@ -20,7 +20,6 @@
 ***********************************************************************/
 
 #include <ast.h>
-#include <assert.h>
 #include <tv.h>
 #include <tm.h>
 #include <error.h>
@@ -55,8 +54,6 @@
 int
 tvsleep(const Tv_t* tv, Tv_t* rv)
 {
-	assert(tv); /* Validate argument */
-
 	/* Return immediately if asked to sleep for no duration. */
 	if (!tv->tv_sec && !tv->tv_nsec)
 		return 0;