Remove duplicated _rand48 implementation. Use stdlib version instead

Originally, there are _rand48 (in libc/bionic/_rand48.c) and __rand48
(in libc/stdlib/_rand48.c) implemented in bionic.  Besides the naming,
the functionality is identical.  This patch removes the duplicated
_rand48.  Also, drand48 and erand48 are modified accordingly.

Change-Id: Ie5761a0a97f45df8538222a77edacb7c3e0125d7
diff --git a/libc/bionic/erand48.c b/libc/bionic/erand48.c
index 843ff34..4ecbead 100644
--- a/libc/bionic/erand48.c
+++ b/libc/bionic/erand48.c
@@ -18,7 +18,7 @@
 double
 erand48(unsigned short xseed[3])
 {
-	_dorand48(xseed);
+	__dorand48(xseed);
 	return ldexp((double) xseed[0], -48) +
 	       ldexp((double) xseed[1], -32) +
 	       ldexp((double) xseed[2], -16);