remove into libc::statx_timestamp conversion for Timespec

Summary:
[#3978](rust-lang/libc#3978) introduced the error bellow:
```
error[E0451]: field `__statx_timestamp_pad1` of struct `statx_timestamp` is private
  --> fbcode/hermetic_infra/reverie/reverie-syscalls/src/args/time.rs:56:13
   |
56 |             __statx_timestamp_pad1: [0],
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^ private field

error: aborting due to 1 previous error
```
Removing `From` impl since it does not appear to be used anywhere.

Reviewed By: zertosh

Differential Revision: D68680975

fbshipit-source-id: f54cb2e2cf1b6a03cda8b8427a7709a2793f2c67

reverie-syscalls/src/args/time.rs

@@ -48,16 +48,6 @@ impl From<libc::statx_timestamp> for Timespec {
     }
 }
 
-impl From<Timespec> for libc::statx_timestamp {
-    fn from(tp: Timespec) -> Self {
-        libc::statx_timestamp {
-            tv_sec: tp.tv_sec as _,
-            tv_nsec: tp.tv_nsec as _,
-            __statx_timestamp_pad1: [0],
-        }
-    }
-}
-
 impl From<libc::timeval> for Timespec {
     fn from(tv: libc::timeval) -> Self {
         Timespec {