Clean up ucontext.h TODOs.

There's no need to expose gregset_t and fpregset_t when they
are not used in ucontext_t.

Bug: 12828904
Change-Id: Ieda1edf0ab18015f444c4d385fe9c41c4544017f
diff --git a/libc/include/sys/ucontext.h b/libc/include/sys/ucontext.h
index f86d9f4..20dcdd5 100644
--- a/libc/include/sys/ucontext.h
+++ b/libc/include/sys/ucontext.h
@@ -60,8 +60,6 @@
 typedef int greg_t;
 typedef greg_t gregset_t[NGREG];
 
-/* TODO: fpregset_t. */
-
 #include <asm/sigcontext.h>
 typedef struct sigcontext mcontext_t;
 
@@ -77,8 +75,6 @@
 
 #elif defined(__aarch64__)
 
-/* TODO: gregset_t and fpregset_t. */
-
 #include <asm/sigcontext.h>
 typedef struct sigcontext mcontext_t;