Remove memset() for SummaryEntry in weighted quantiles. This is a bug whenever...

Remove memset() for SummaryEntry in weighted quantiles. This is a bug whenever one of the template arguments is not a POD, in particular for types such as string. A memset on a struct that contains a string invalidates the internals of that class (in particular the pointer that stores the characters of the string) and causes segfaults. This allows using weighted quantiles beyond int/float like template parameters. The upstream issue with memcopy of alignment bytes used for padding has been solved separately in the meantime.

PiperOrigin-RevId: 209906298