AK: Remove StringBuilder::build() in favor of to_deprecated_string()

Having an alias function that only wraps another one is silly, and
keeping the more obvious name should flush out more uses of deprecated
strings.
No behavior change.

Userland/Libraries/LibC/stdlib.cpp

@@ -641,7 +641,7 @@ int ptsname_r(int fd, char* buffer, size_t size)
         return -1;
     }
     memset(buffer, 0, devpts_path_builder.length() + 1);
-    auto full_devpts_path_string = devpts_path_builder.build();
+    auto full_devpts_path_string = devpts_path_builder.to_deprecated_string();
     if (!full_devpts_path_string.copy_characters_to_buffer(buffer, size)) {
         errno = ERANGE;
         return -1;