[lldb] Remove forgotten FIXME on CPlusPlus formatters

The patch [1] introduced this FIXME but ended up not being removed when fixed.

[1]: https://github.com/llvm/llvm-project/commit/f68df12fb039d5177e34f4541fa242b891949db6



Signed-off-by: Luís Ferreira <contact@lsferreira.net>

Reviewed By: teemperor

Differential Revision: https://reviews.llvm.org/D112586