Revert "std.c: openbsd sigcontext/ucontext for arm64."

This reverts commit 1585ed637d.

lib/std/c/openbsd.zig

@@ -1127,18 +1127,6 @@ pub usingnamespace switch (builtin.cpu.arch) {
             sc_cookie: c_long,
         };
     },
-    .aarch64 => struct {
-        pub const ucontext_t = extern struct {
-            __sc_unused: c_int,
-            sc_mask: c_int,
-            sc_sp: c_ulong,
-            sc_lr: c_ulong,
-            sc_elr: c_ulong,
-            sc_spsr: c_ulong,
-            sc_x: [30]c_ulong,
-            sc_cookie: c_long,
-        };
-    },
     else => struct {},
 };