Revert "libc: Provide ucontext_t/mcontext_t/<sys/ucontext.h>"

This creates build issues in the internal Android tree.
Will investigate later.

Original patch: https://android-review.googlesource.com/#/c/38875/

Change-Id: I12c5995ebf172890051af42a5d3b31014c9c5117
diff --git a/libc/include/time.h b/libc/include/time.h
index f65ac4e..e280e0a 100644
--- a/libc/include/time.h
+++ b/libc/include/time.h
@@ -30,7 +30,10 @@
 
 #include <sys/cdefs.h>
 #include <sys/time.h>
-#include <machine/signal.h>  /* for struct sigevent */
+
+#define __ARCH_SI_UID_T __kernel_uid32_t
+#include <asm/siginfo.h>
+#undef __ARCH_SI_UID_T
 
 __BEGIN_DECLS