ctime: Drop 32-/64-bit distinction on Linux

We added the explicit int64 casts for 32-bit builds in 35e1ad78 (Make
libpod build on 32-bit systems, 2018-02-12, #324), but the explicit
casts work fine on 64-bit systems too.

Signed-off-by: W. Trevor King <wking@tremily.us>

Closes: #1058
Approved by: mheon

pkg/ctime/ctime_linux.go

@@ -1,4 +1,4 @@
-// +build arm,linux 386,linux
+// +build linux
 
 package ctime
 

pkg/ctime/ctime_linux_64.go

@@ -1,14 +0,0 @@
-// +build !arm,!386,linux
-
-package ctime
-
-import (
-	"os"
-	"syscall"
-	"time"
-)
-
-func created(fi os.FileInfo) time.Time {
-	st := fi.Sys().(*syscall.Stat_t)
-	return time.Unix(st.Ctim.Sec, st.Ctim.Nsec)
-}