AK: Don't allow Vector::resize_and_keep_capacity() in the kernel

This function doesn't propagate allocation failures.

Replace the only current user of this function by using `shrink()` with
`keep_capacity = true` instead.

AK/Stack.h

@@ -49,7 +49,7 @@ public:
         if (is_empty())
             return false;
 
-        m_stack.resize_and_keep_capacity(m_stack.size() - 1);
+        m_stack.shrink(m_stack.size() - 1, true);
         return true;
     }
 

AK/Vector.h

@@ -788,13 +788,13 @@ public:
     {
         MUST(try_resize(new_size, keep_capacity));
     }
-#endif
 
     void resize_and_keep_capacity(size_t new_size)
     requires(!contains_reference)
     {
         MUST(try_resize_and_keep_capacity(new_size));
     }
+#endif
 
     void shrink_to_fit()
     {