[libc++] Remove more unnecessary _VSTD:: from type names. NFCI.

Differential Revision: https://reviews.llvm.org/D102181

NOKEYCHECK=True
GitOrigin-RevId: 6491d99e330c38b33b9cb6acb19afa3a464febeb
diff --git a/include/random b/include/random
index eb2d37d..f2eaf84 100644
--- a/include/random
+++ b/include/random
@@ -6292,8 +6292,7 @@
         if (__p_.size() > 1)
         {
             double __s = _VSTD::accumulate(__p_.begin(), __p_.end(), 0.0);
-            for (_VSTD::vector<double>::iterator __i = __p_.begin(), __e = __p_.end();
-                                                                       __i < __e; ++__i)
+            for (vector<double>::iterator __i = __p_.begin(), __e = __p_.end(); __i < __e; ++__i)
                 *__i /= __s;
             vector<double> __t(__p_.size() - 1);
             _VSTD::partial_sum(__p_.begin(), __p_.end() - 1, __t.begin());
@@ -6312,7 +6311,7 @@
 discrete_distribution<_IntType>::param_type::probabilities() const
 {
     size_t __n = __p_.size();
-    _VSTD::vector<double> __p(__n+1);
+    vector<double> __p(__n+1);
     _VSTD::adjacent_difference(__p_.begin(), __p_.end(), __p.begin());
     if (__n > 0)
         __p[__n] = 1 - __p_[__n-1];