Kernel/LibC: remove PATH resoltion from kernel

I have no idea why I had made PATH environment variable parsing
to be part of the kernel. Now the shell does the parsing and
environment syscall is no longer needed.

libc/include/sys/syscall.h

@@ -21,7 +21,6 @@ __BEGIN_DECLS
 #define SYS_SLEEP 17
 #define SYS_WAIT 18
 #define SYS_FSTAT 19
-#define SYS_SETENVP 20
 #define SYS_READ_DIR_ENTRIES 21
 #define SYS_SET_UID 22
 #define SYS_SET_GID 23