[namespace.alias], [over.oper.general] Remove trailing \br in grammar

[eisenwave]

Using

pcregrep -M -n -r '\\br\s+\\end' *.tex

I was able to find a few bugs where \br was added at the end of a grammar. This results in a bogus line break:

These are the only two such occurrences.

We should also make a check that prevents this from happening again: #8494

[tkoeppe]

Thank you, yes, this is quite a common mistake, and if we could check for it, that'd be helpful.