[lldb][docs][NFC] Remove references to obsolete gnu-libstdc++ category (#141610)

This is still leftover from the days when the libc++ and libstdc++
formatters were both written in python and in separate categories. Since
then we group libstdc++ and libc++ formatters into the same cateogry.

This patch removes references to the obsolete `gnu-libstdc++` category
from the docs (and a test).

See [this
thread](https://github.com/llvm/llvm-project/pull/140761#discussion_r2102386080)
for more context

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