libast: cleanup: remove obsolete exec(3) fallbacks

execlp, execve and execvp have all been in POSIX since Issue 1, so
the need for those fallbacks expired many years ago.

execvpe is a GNU extension, so that fallback is not obsolete --
however, it is unused (even in the full original AST code base), so
let's remove that as well.

src/lib/libast/Mamfile

@@ -2601,40 +2601,6 @@ make install virtual
 			exec - compile ${<}
 		done
 
-		make execlp.o
-			make comp/execlp.c
-				prev include/ast.h
-				prev ast_lib.h
-			done
-			exec - compile ${<}
-		done
-
-		make execve.o
-			make comp/execve.c
-				prev include/error.h
-				prev include/wait.h
-				prev sig.h
-				prev include/ast.h
-			done
-			exec - compile ${<}
-		done
-
-		make execvp.o
-			make comp/execvp.c
-				prev include/ast.h
-				prev ast_lib.h
-			done
-			exec - compile ${<}
-		done
-
-		make execvpe.o
-			make comp/execvpe.c
-				prev include/ast.h
-				prev ast_lib.h
-			done
-			exec - compile ${<}
-		done
-
 		make spawnveg.o
 			make comp/spawnveg.c
 				prev ast_tty.h

src/lib/libast/features/lib

@@ -24,7 +24,6 @@ dat	_tzname,tzname
 lib	BSDsetpgrp
 lib	_cleanup
 lib	bcopy,bzero,confstr,dirread,dup2
-lib	execlp,execve,execvp,execvpe
 lib	fchmod,fcntl,fmtmsg,fnmatch,fork,fsync
 lib	getconf,getdents,getdirentries,getdtablesize,getdate
 lib	getgroups,gethostname,getlogin,getpagesize,getrlimit,getuniverse

src/lib/libast/features/map.c

@@ -70,11 +70,6 @@ main(void)
 #if !_lib_eaccess
 	printf("#undef	eaccess\n");
 	printf("#define eaccess		_ast_eaccess\n");
-#endif
-#if !_lib_execvpe
-	printf("#undef	execvpe\n");
-	printf("#define execvpe		_ast_execvpe\n");
-	printf("extern int		execvpe(const char*, char* const[], char* const[]);\n");
 #endif
 	printf("#undef	fnmatch\n");
 	printf("#define fnmatch		_ast_fnmatch\n");