[libc] remove a few unused header files

strings.h and alloca.h are unused and a little funny anyway. Probably
were once used in an external library that's no longer in the tree.

lib/libc/include/alloca.h

@@ -1,6 +0,0 @@
-#if !defined(__ALLOCA_H)
-#define __ALLOCA_H
-
-#define alloca(size) __builtin_alloca (size)
-
-#endif  /* !__ALLOCA_H */

lib/libc/include/strings.h

@@ -1,12 +0,0 @@
-#if !defined(__STRINGS_H)
-#define __STRINGS_H
-
-#include <lk/compiler.h>
-
-__BEGIN_CDECLS
-
-int strcasecmp(const char *s1, const char *s2);
-
-__END_CDECLS
-
-#endif  /* !__STRINGS_H */