Revert [60ac25a4/lyxgit], [30f16463/lyxgit], and part of [e8f480e7/lyxgit]

Revert [60ac25a4/lyxgit], [30f16463/lyxgit], and part of [e8f480e7/lyxgit]

As evidenced in #11552, at loading time there is no way to tell
whether braces were added by LyX with certainty. However, after
[503f7db2/lyxgit], LyX does not automatically removes brace insets.
So, in case one faces the problems mentioned in [e8f480e7/lyxgit],
the workaround of adding a brace inset now will not be frustrated.
In essence, after [503f7db2/lyxgit], the real fix for #11552 would
have been reverting the part in [e8f480e7/lyxgit] that was adding
the braces on output.