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/arch-mips/bionic/setjmp.S b/libc/arch-mips/bionic/setjmp.S
index 0b1538c..7c21195 100644
--- a/libc/arch-mips/bionic/setjmp.S
+++ b/libc/arch-mips/bionic/setjmp.S
@@ -31,7 +31,7 @@
 
 #include <machine/asm.h>
 #include <machine/regnum.h>
-#include <machine/setjmp.h>
+#include <machine/signal.h>
 
 /*
  * setjmp, longjmp implementation for libc. this code depends